REDDISH GREEN

A CHALLENGE FOR MODAL CLAIMS ABOUT PHENOMENAL STRUCTURE1

Martine Nida-Rümelin & Juan Suarez

Université de Fribourg, Switzerland

Abstract: We discuss two modal claims about the phenomenal structure of color experiences: (i) violet experiences are necessarily experiences of a color that is for the subject on that occasion phenomenally composed of red and blue (the modal claim about violet) and (ii) no subject can possibly have an experience of a color that is for it then phenomenally composed of red and green (the modal claim about reddish green). The modal claim about reddish green is undermined by empirical results. We discuss whether these empirical results cast doubt on the other modal claims as well. We argue that this not the case. Our argument is based on the thesis that the best argument for the modal claim about violet is quite different from the best argument for the modal claim about reddish green. To argue for this disanalogy we propose a reconstruction of the best available justification for both claims.

1. Violet and Greenish Red. Introduction

Most people agree that violet appears to be composed of red and blue. Violet

experiences are experiences of a binary hue, a hue that is experienced as composed of

red and blue. It is common to talk in this context of phenomenal composition in order to

be clear that the claim is about the appearance of colors and not about their physical

composition. Certain hues can combine in experience into a phenomenally composed

color. Like blue and red, yellow and red can combine in experience into a phenomenally

composed color (orange) and the pair of hues green and blue can combine in

experience in this way too. However, there are hues that – at least under normal

circumstances – are never combined in experience into a phenomenally composed

color. This is true for the pair of red and green and for the pair of yellow and blue. For a

long time it has been accepted that not only we never see a reddish green or a yellowish

blue under normal circumstances but rather that it is in principle impossible to have an

experience of a color that appears composed of red and green or of yellow and blue.

According to this modal claim it is part of the nature of the colors (or rather of the hues)

at issue that they cannot combine in experience in the way blue and red can.

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This quite natural modal assumption has however been challenged by experimental

results conducted by Crane and Piantanida and by Billock, Gleason, and Tsou.2 If taken

at face value, these experiments show that it is possible for human beings to see a color

that has a reddish and a greenish component. We assume that what is at issue here is

the question whether red and green can combine in experience to yield a new

experienced color that is phenomenally composed of red and green in the way violet is

composed of red and blue or orange is composed of red and yellow.

We will discuss whether the experimental data just mentioned cast doubt on the modal

claim that it is in principle impossible to see a greenish red. If so, then it is natural to

generalize the doubt and to argue that modal claims that may appear to be a priori

knowable about the nature of color and color experience have been shown to be

dependent upon empirical assumptions that need to be tested by experiments. In our

discussion of the latter general issue we propose an epistemological account of modal

knowledge about the necessary phenomenal structure of color experience.

2. Preliminary remarks about Phenomenal Composition

To formulate the philosophical issues we are interested in will require using the concept

of phenomenal composition.3 Before we begin we have to say a few words about how

we understand the concept and we have to distinguish two different notions of

phenomenal composition.

(1) We will assume that the claim

(V) Violet is composed of red and blue

is made true by a particular aspect of the subjective character of violet experiences. It is

not made true, in our view, by any fact concerning the mixture of pigments or the mixture

of light. (V) is true because things that look violet to a person look to that person in a

particular way: they look reddish and bluish and there is no other chromatic hue

component present in the color as it is perceived by the subject at issue. You may insist

that (V) may be interpreted as a claim about how to mix violet with pigments on a piece

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of paper. This may be true but this possible interpretation of (V) is not the interpretation

we will be interested in. There is a reading of (V) – or so we claim – that is made true by

phenomenal facts. This is why we call the relation stated in (V) the relation of

phenomenal composition.

(2) Sentences like (V) about the phenomenal composition of a given color do not

mention specific people or circumstances. They are naturally understood as sentences

about all possible violet experiences by arbitrary subjects in arbitrary circumstances.

Understood in this way (V) is a modal claim made true by necessary phenomenal facts.

There might be an admissible interpretation of (V) that is made true by non-modal facts

about all e.g. human color experiences. If so, this is not the interpretation we will be

interested in. In the relevant modal interpretation of (V) this sentence is true because

violet is necessarily experienced as being composed of red and blue. According to (V), if

a subject has an experience of a color that does not contain any reddish or bluish

component then the color experienced cannot be violet. In other words: no subject in

actual or counterfactual circumstances has an experience of something as violet without

thereby having an experience of something as looking bluish and reddish at the same

time. So, according to the interpretation of (V) that we will presuppose, (V) implies that it

is constitutive of having an experience of violet that it is an experience of a color that is

seen as containing a reddish and a bluish component.

(3) Sceptics about modal knowledge will be tempted to interpret claims like (V) about the

phenomenal composition of a color as true by definition.4 They will argue that (V) and

similar claims are true by linguistic convention. In our view, this is a misunderstanding. In

the claim (V) a particular phenomenal kind of experience is picked out by the term ‘violet’

and then a substantial assertion is made about the phenomenal structure of experiences

of this kind. Different people can disagree about the phenomenal composition of a color

without thereby showing any kind of linguistic failure. Brentano, who famously believed

that green is phenomenally composed of blue and yellow, made a substantial mistaken

assumption about a particular kind of color experiences humans are acquainted with. He

could not have been corrected by simply stating that he was misusing a linguistic term.

(4) The modal claim that violet experiences are necessarily such that the color at issue is

experienced as reddish and bluish simultaneously has sometimes been invoked to argue

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that being phenomenally composed of red and blue is a necessary property of the color

violet.5 In our view it is important – in order to avoid confusion – to distinguish in this

context conceptually between modal claims about colors and modal claims about the

structure of color experiences. It seems clear that we cannot understand the modal claim

that violet is necessarily composed of red and blue without a prior understanding of the

simpler claim that violet is composed of red and blue. If our view about claims like (V) is

correct then sentences of this kind are naturally understood as modal claims about color

experiences. It follows that the corresponding modal claim about the color violet is a

second order modal claim: to say that violet is necessarily composed of red and blue is

to say that it is a necessary property of the color violet that experiences of this color

have a specific necessary structure. This further modal claim appears plausible to us.

So we would like to stress that assertions like (V) are already (although they do not yet

explicitly contain a modal operator) modal claims about violet experiences. We do not

wish to contradict the quite common view that an assertion like (V) expresses a truth

about the color violet that is necessarily true of violet. However, this further modal claim

about the color violet should not be confused with the modal claim about violet

experiences expressed by (V) itself.

(5) What we said about (V) in the preceding sections (1)-(4) cannot be expressed

without using two distinct notions of phenomenal composition. The first notion of

phenomenal composition is applied to colors (or hues) and does not involve explicit

reference to people. It is the notion that occurs explicitly in the sentence (V). The notion

is used to attribute a property to violet (the property of being composed of red and blue).

The second notion has been used in the preceding remarks to explain the content of (V).

We said that (V) expresses a modal claim about the structure of individual concrete color

experiences. To express this modal claim about the structure of individual experiences

we need a notion of phenomenal composition that simply describes the structure of

individual experiences in a way that does not yet contain any modal judgement. This is

where the second notion of phenomenal composition must be introduced. This second

notion is used to say about a concrete color experience that it is such that the subject at

issue sees a reddish and a bluish component. To say this of a particular experience is to

describe the experience at issue in isolation without thereby making any assumption

about the structure or necessary structure about experiences of the relevant kind in

general. According to the view about (V) sketched before, the first notion of phenomenal

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composition is to be explained in terms of the second: to be phenomenally composed in

a specific way (first sense) is, for a color, to be such that experiences of this color

necessarily have a specific phenomenal structure (the description of this structure is

accomplished by the second notion of phenomenal composition). The failure to

distinguish these two notions of phenomenal composition would make it impossible to

express the content of assertions like (V) in a coherent and non-circular manner.6

It is not easy to find a satisfactory verbal expression for the second notion. We will

sometimes use the following locution: "the object appears to the person in a color that is

phenomenally composed of red and blue for that person on that occasion" or we will

directly speak of the relevant components using terms like “reddish” or “bluish”. When

using the latter locution we will simply say that an object appears e.g. reddish and bluish

to a specific person on a given occasion.

For our philosophical purposes it is important to be able to give phenomenological

descriptions of concrete color experiences that do not imply modal claims about color

experiences of the relevant type. It must be possible e.g. to claim that for Mary

something that looks violet to her on a particular occasion also looks reddish and bluish

to her without thereby implying that what is said about the phenomenal structure of

Mary’s particular experience has to be true for any possible violet experience as well. It

is quite easy to see that this constraint cannot be met without the conceptual distinction

that we just introduced.7

(6) With respect to the second notion of phenomenal composition it is important to note

that we do not exclude the possibility of error by the experiencing person herself

concerning her own experience. A person may have an experience of a unique hue (e.g.

green) and yet mistakenly judge that she experiences that hue as binary (e.g. as

composed of blue and yellow).8 A person may actually see a reddish blue and yet not

notice that the specific blue she experiences does look reddish to her. According to this

thesis we can be wrong about facts concerning phenomenal composition (in the sense

of the second notion) even with respect to our own color experiences and this even while

having them and even while directing our attention towards the relevant phenomenal

aspects. According to this view, to see a color as composed in a particular way is not to

be equated with judging or being disposed to judge that there are certain components

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present in one’s own color experience. This fallibilist claim is part of a view we call

realism with respect to phenomenal composition in other places.9 We use the term

‘realism’ to hint to the idea that claims about phenomenal composition concerning

specific concrete experiences are made true by facts concerning the subjective

character of the experience at issue and that obtain independently of the way they are

judged by the person concerned or by other people and independently of linguistic

conventions about the use of color terms.

(7) We use talk of phenomenal composition of a color (of e.g. blue and red) to describe

phenomenal facts, e.g., that a person sees a color as reddish and bluish simultaneously.

It should be clear, however, that the term ‘composition’ should not be taken too seriously

in this context. There are not literally two ‘components’ involved. It would be a mistake to

conclude that violet experiences (and other experiences of binary hues) are in any clear

sense more complex than red experiences (and other experiences of unique hues). It

would be a mistake to ask for an account of the relation of composition that is apparently

invoked. We do not wish to state that colors that appear composed in the relevant sense

literally appear to contain two distinct ‘phenomenal parts’. It would be phenomenologi-

cally inadequate to think of experiences of a reddish and bluish color as an experience

of two separate components that are in some clear sense co-present in the experience

(like two tones may be literally co-present in an experience of an interval). Unfortunately

talk of ‘components’ or composition in this context invites a number of mistaken ideas

about the nature of the phenomenal fact thus referred to.10 But since there is no other

introduced term to be found in our common or more technical vocabulary to express the

notion at issue we will have to go on talking of ‘composition’ and ‘components’ to

describe that particular phenomenal aspect.

3. Modal Knowledge about Phenomenal Composition. A Problem Raised by Experimental Data about Reddish Greens If claims like (V) are correctly interpreted as modal claims about the phenomenal

character of violet experiences then the following epistemological question arises: How

can we know that violet experiences have a certain phenomenal structure necessarily?11

It appears to be quite unproblematic to assume that a person can know – by carefully

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attending to her own violet experience while having it – that she is thereby having an

experience of a color that is for her phenomenally composed of red and blue on that

occasion. But how can she know that this will be so again next time when she will have a

violet experience? And how can she know that no subject can ever have an experience

of violet without thereby having an experience of a color that is for the subject concerned

phenomenally composed of red and blue on that occasion? How can a person rationally

judge about violet experiences at other times in other galaxies had by different sentient

beings? Or is it maybe irrational to hold the modal claim that is implied by claims like

(V)?12

We will argue that it is possible to rationally judge on the basis of one’s own experiences

that (V) is true. Given our interpretation of (V) this implies the claim that we do have

epistemic access to modal truths about phenomenal composition (in the second sense

of the notion). We will defend the view that a person who (a) judges on the basis of

carefully attending to her own violet experiences that she has an experience of a color

that is phenomenally composed of red and blue for her whenever she has an experience

of this type, does indeed have reason to (b) judge that having an experience of a reddish

and a bluish component is indeed constitutive of having an experience of violet. This

view involves the claim that we can rationally proceed from phenomenological

observations about our own experiences to modal claims about experiences of the

relevant type in general. How can we do this? What is the correct reconstruction of the

steps involved in a justification of the modal claims at issue? We will propose a

justification of modal claims about phenomenal composition (in the second sense of the

term) in a moment. But there is a potential danger for our view based on the case of

reddish greens. Let us have a look at this danger.

In certain cases, we claim, we can rationally form judgements about the necessary

structure of color experiences in general on the basis of phenomenological descriptions

concerning the phenomenal structure of our own actual color experiences. This is the

view we defend and the case we cited is the one about violet. However, the case of

reddish greens seems to show that we are easily led astray when we dare to proceed

from phenomenological descriptions of phenomenal structure to modal claims about

phenomenal structure. It is a true and quite obvious phenomenological fact about the

phenomenal structure of normal human color perception that we never see a color that is

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for us phenomenally composed of red and green. If we try to imagine seeing something

as reddish and greenish simultaneously then we simply do not succeed. In normal

human color perception red and green are opponents in Hering's sense: there is no hue

ever seen by a real person in real life that is phenomenally composed of red and green

for that person on that occasion. We are tempted to conclude, like in the case of violet,

that this particular fact about the actual structure of our color experience is a necessary

truth about red and green. We are tempted to say something like this: if some arbitrary

subject under counterfactual circumstances sees a color that is for that subject

phenomenally composed of two hues H1 and H2, then these hues are not green and

red. It is – so it seems – constitutive of red and green that they cannot be combined in

experience to yield a new hue in the way red and blue or red and yellow or blue and

green can.13

But the empirical results mentioned before seem to show that this view must be

abandoned and they cast doubt on the claim that we have modal knowledge about

phenomenal structure in the case of violet and orange as well. Our modal judgement in

the case of violet is based on knowledge about the phenomenal structure of our actual

experiences. Something very similar seems to be going on in the case of the apparently

wrong modal judgement about red and green. Maybe then we should admit that we

simply do not have anything more than knowledge about actual phenomenal structure

and maybe we should withdraw opinion with respect to the corresponding modal claims

in general. To see whether this is the conclusion to draw we will first have a look on the

relevant empirical results.

4. Empirical studies about reddish green

In a study by Crane and Piantanida14 subjects reported seeing reddish-greens and

yellowish-blues. A variant of this experiment has been successfully conducted by Billock

et al.15 To our knowledge no other similar experiments have been realized. The

conditions under which the experiences occurred that were described by the subjects

concerned as experiences of reddish green or of bluish yellow were highly unnatural. It

is therefore unlikely that experiences of these so-called ‘novel colors’16 ever occur

outside a vision science lab. Some features of these experiments that are relevant to our

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purposes require some introduction to non-scientists. In this section we will therefore

introduce some of the relevant technical terms and then describe the experiment.

4.1. A few technical terms

Stabilized images and filling in

The eye is rarely stationary for more than a few microseconds. In order to see, the eye

needs contrast, which is provided by constant, very fast movements of the eye called

saccades. This mechanism probably enables us to avoid seeing the blood vessels

placed between the retina and the visual scene.17 If the eye totally lacks contrast for a

few seconds then the image will fade out. This is what happens in the so-called ganzfeld

phenomenon, in which the contrast provided by the visible parts of the subject’s face is

hidden from view. C L Hardin describes an example of this phenomenon:

"A subject’s eyes are covered with the two halves of a ping-pong ball and the room is filled with a diffuse, shadowless colored light, pink for example. Within a couple of minutes’ time, the color fades completely from view; subjects frequently claim that the light has been turned off." (Hardin, 1993) p.22

A stabilized image is an image that is projected on a part of the visual field and which

follows the movements of the eye, so that the fading out of the image typical of the

ganzfeld phenomenon is restricted only to the stabilized portion of the visual field. This

can be done with contact lenses with a non-transparent part. In the experiments at issue

the stabilization is done with an ‘eyetracker’, a device designed to stabilize images

without using contact lenses with non-transparent parts. Eyetrackers use the reflection of

infrared light of the eye to track its movements and project images onto it.18 If an image

is stabilized on a part of the retina for a certain time, thus producing a sort of

'informational hole' underlying the fading out phenomenon just described, then the brain

tends to complete the image by filling in something using the information of the surround.

The filling-in process is commonly taken to explain the fact that there is no perceived

hole in our visual field corresponding to the blind spot on the retina. The blind spot is the

place in the retina where the optic nerve exits the eye. In this part of the retina there are

no photoreceptors, and therefore no visual information is received, and yet we do not

notice this informational hole except in very exceptional circumstances.19 Scotomas

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(lesions of the retina) can go unnoticed by the subject even when much larger than the

blind spot. This again is considered to be explainable by a filling in process.

Equiluminance

There are three types of photoreceptors that participate in color vision, individuated by

their sensitivity to different wavelengths. The S-cones are maximally sensible to short

wavelengths, the M-cones to middle wavelengths and the L-cones to long wavelengths.

According to a widely accepted model the ‘luminance pathway' processes the sum of the

activity of the L- and M- cones.20 It is assumed that two perceived surfaces are

equiluminants when the degree of activity in the luminance pathways produced by the

surfaces is the same. In psychophysics equiluminance is usually defined operationally:

when under certain circumstances the visibility of, e.g., the border between two different

coloured surfaces is minimized the two surfaces are called equiluminants. Because of

the inter-personal differences of sensitivity to wavelengths even between normal

subjects, two surfaces are equiluminants only relative to a subject, and the

equiluminance of two surfaces must be determined for subjects individually.

Equiluminance between two chromatically different parts of the visual field tend to

produce perceptions of ‘problematic borders’, fading borders and a series of chromatic

illusions. By mixing stabilized images to equiluminance, the likelihood that the borders of

the contours of the image fade are increased.

4.2. The experiment

There were 7 subjects in the experiment of Billock et al, the exact number of observers

participating in the experiment of Crane and Piantanida is not available.21 The subjects

were presented with a red and a green stripe on a black field, such that the red and

green stripes had a common border. In both experiments the red-green field was

stabilized using an eyetracker. This was done in order to provoke a filling-in process in

which the information from the non-stabilized parts of the image (the edges between the

black field and the red and green stripe) should be used. The idea is that the subjects

perceive the outer edges of the red and green stripes but not the common red-green

edge in the middle (see figs. 1 and 2 below).

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In the experiments conducted by Billock et al. the two stripes were mixed to

equiluminance for each subject. Here is a list of the kinds of reports that were obtained

in the two experiments. Each of the following descriptions occurred in both experiments

but the exact numbers of subjects are given only in Billock et al. We add the latter

numbers in parentheses.

(1) Homogenous mixture: Subjects report seeing a homogeneous color phenomenally

composed of red and green whose components are as clear and as compelling as the

red and blue components of a purple.22 (4 subjects out of 7 in Billock et al.)

(2) Gradient color: Subjects report seeing a gradient color that runs from red on one

side to green on the other with a large region in between that appears both red and

green. (5 subjects out of 7 in Billock et al.)

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(3) Special arrangements of the visual field: Subjects report seeing either (a) two

color stripes that seem to switch sides rapidly before fading out or regaining their places.

(4 subjects out of 7 in Billock et al) Or they report seeing (b) the red and green stripes

arranged horizontally rather than vertically (1 subject out of 7 in Billock et al.)

(4) Blackouts: Subjects report that the visual field turns into black (like in the ganzfeld

phenomenon). (6 subjects out of 7 in Billock et al.)

(5) Transparency phenomenon: Subjects report that it looks as though the opponent

colors originate in two depth planes and as though they are seen one trough the other.

(4 subjects out of 7 in Billock et al.)

(6) Dots: Subjects report that the visual field appears to be composed entirely of a

regular array of just resolvable red and green dots. (Number of subjects not available)

(7) Islands: Subjects report that the visual field appears as a number of islands of one

color on a background of the other color. (Number of subjects not available)

In almost each trial the subject reported more than one of the kinds of experiences

described in (1) to (7). In most trials where subjects reported having had experiences of

the kind (1) and (2), they also reported that these experiences were preceded by

experiences of the kind described in (4) and (5). The hypothesis driving the experiments

of Billock et al. is that experiences such as those reported in (1) and (2) are more likely

to occur if the two stripes are equiluminant. This hypothesis was motivated by the

findings of (Nerger, Piantanida, & Larimer, 1993) who had shown that differences in

luminance may lessen the effects of filling in. Several trials increased the probability of

reports like (1). Two subjects reported independently after the experiment that “reddish-

green and yellowish-blue colors could now be imagined.” (Billock et al., 2001) p.2399

Six of the seven subjects of Billock et al.’s study were expert psychophysicists, including

some researchers in vision. Crane and Piantanida participated as subjects in their own

experiment and so did two of the authors in Billock et al in their new version. Even

though the experimenters made efforts to ensure that the subjects had a large color

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vocabulary, several subjects seem to have had difficulties in describing the phenomenon

they had experienced as is documented by the following two citations:

"Although most [of the subjects] reported that regardless of where they attended in the field the color was simultaneously both red and green, some observers indicated that although they were aware that what they were viewing was a color (that is, the field was not achromatic), they were unable to name or describe the color. One of these observers was an artist with a large color vocabulary." (Crane & Piantanida, 1983) p.1079 "

“Our subjects (like those in (Crane & Piantanida, 1983)) were tongue-tied in their descriptions of these colors, using terms like “green with a red sheen” or “red with green highlights.” (Billock et al., 2001) p.2398 [Here the authors are referring to reports (2), (3), (5), (6), (7), above].

5. Current Interpretations of the Empirical Result 5.1. Taking the subjects reports at face value To take the subject reports of Crane and Piantanida and Billock et al.’s studies at face

value is to take these reports as literally and accurately describing the experiences the

subjects were reporting. Those subjects who gave reports like (1) are thus taken to have

had experiences as of binary colors with a reddish and a greenish component in the

case of reddish-greens and with a bluish and a yellowish component in the case of

yellowish-blues.

Here are a few at least prima facie reasons that can be advanced in favour of this

interpretation:

(i) The experiment of Crane and Piantanida has been repeated successfully by Billock et

al. and it has been modified in a way that increases the likelihood of reports like (1) and

(2).

(ii) The only existing empirical theory which predicts the impossibility of novel colors,

standard opponent process theory, is controversial in its original unmodified form and

open to different interpretations that need not all imply the impossibility of experiences of

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reddish greens.23 According to the models proposed by the authors of the two

experiments perception of novel colors is expected under some special circumstances.

(iii) Some of the subjects, including Piantanida and Billock, are vision scientists and are

expected to be experts in the use of color vocabulary. This is a good reason to take their

reports at face value.

(iv) The list of different reports given by the subjects supports the idea that the subjects

were trying to be precise in their description and to avoid ambiguities.

5.2. Scepticism about novel colors This section discusses the possibility of not taking the subject reports at face value and

some reasons for doing so.

Crane and Piantanida’s findings were quite controversial at the time of their

publication.24 This was in part due to the fact that only some of Crane and Piantanida’s

subjects reported seeing novel colors. In the absence of other evidence, or a compelling

explanation of why only some of the subjects in the experiment reported having seen a

binary hue composed of red and green (or blue and yellow), it seems reasonable to be

careful about these reports. Another source of doubt is the reluctance of the subjects to

describe the colors as reddish-green. Part of this reluctance may be explained by the

novelty of the experience and the fact that the subjects may have been prejudiced

against the possibility of novel colors. But the reluctance may also be explained by the

quality of the experiences itself, which some of the subjects of Crane and Piantanida’s

study described later to Larry Hardin as being dark and muddy, adding that these two

qualities make colors hard to judge.25

At least some of the sceptical worries are answered in Billock et al.’s study. First, there is

a tentative explanation of why only some of the subjects of Crane and Piantanida’s study

reported seeing reddish-greens: in order to see novel colors the stripes have to be

equiluminants and some training may be needed.26 Four out of seven of Billock et al.’s

subjects reported (1) and, according to Vince Billock “the colors we are talking about are

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not muddy and the subjects have rich perceptual vocabularies. The only remaining

reasons for having difficulties describing the effects are that the phenomena are novel

and dynamic (multistable).”27

6. A further doubt about reddish greens (a speculation)

According to the realist view about phenomenal composition that we defend a person

can seriously judge that he or she has an experience of a hue that is phenomenally

composed for her at that moment in a particular way and yet thereby be wrong about the

phenomenal character of his or her own experience. For someone who accepts this view

that explicitly endorses fallibilism about phenomenal judgements it is quite natural to

consider the possibility that those subjects who described the color perceived in the

laboratory as being composed of red and green for them on this occasion were

misdescribing their own experience. In this section we briefly describe a way in which

they might have gone wrong. The possibility we consider is highly speculative, but the

speculation might illustrate the following more general point: even in the apparently

simple case of phenomenal composition of colors it can be very difficult to find an

adequate and unbiased phenomenological description. The error theory we will describe

is not meant to be a serious proposal of an alternative interpretation of the empirical

results. To be a serious proposal it would have to be tested by further especially

designed experiments. The error at issue – according to the present speculation – is a

confusion of two phenomenological descriptions: apparent simultaneous presence of red

and green on a surface is mistakenly described as genuine phenomenal composition of

red and green. According to this speculative proposal there is a possible experience that

is in fact produced under the unusual circumstances at issue that can adequately be

described as a case where red and green are simultaneously present on a surface (for

the perceiver on that occasion) but where red and green do not phenomenally mix into a

new kind of hue as they do in genuine cases of phenomenal composition.

But how can red and green be simultaneously present on a surface in a given

experience without phenomenally mixing into a new hue? What is the kind of experience

like that we claim to be appropriately described in this specific way? Our aim in the

following is twofold: we would like to communicate the phenomenal character we have in

mind (the phenomenal character of those experiences that can be described quite

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appropriately as involving simultaneous presence without phenomenal composition) and

we would like to make it plausible to the reader (even if he or she finds it hard or

impossible to imagine an experience of the relevant kind) that an experience of the kind

at issue may occur under unusual circumstances.

As a first step one should imagine what subjects in the experiments meant when they

described their experiences as being of two layers, one green, and the other red, one

behind the other, “as if looking through a green glass on a red surface”. According to this

description the two surfaces appeared to be located at different distances and the hues

of the two layers did not mix into one as they do in phenomenal composition. Now – as a

second step – let us try to imagine what would happen if the apparent difference in

location were to disappear, all other qualitative aspects remaining the same? Is there

some possible experience that is adequately described in this way: it is – with respect to

hue – like an experience of looking through a green glass at a red surface but the two

surfaces do not appear to be at different places? If so, then – with respect to an

experience of this kind – we could probably say with equal justification that the

experience is like looking through a red glass at a green surface. Furthermore, the

experience might not just be neutral or oscillating between being like ‘looking through a

green glass on a red surface’ and being like ‘looking through a red glass on a green

surface’, rather, there would not be any sense in talking of two surfaces in the

description of the experience at all. Given that any experience of a difference in location

has disappeared, there is only one apparent surface present in the experience. But still

green is on the surface and red is on the surface too. However red and green are there

(apparently) on the surface without mixing into one new color.

These considerations might make it plausible to the reader that there are experiences

that can appropriately be described as involving simultaneous presence of two hues on

one surface without phenomenal composition and they might make the relevant

experience at least partially imaginable to a certain extent. If so, then we have given an

example of how even in this apparently simple case phenomenal descriptions might

easily go wrong. The difference between the two kinds of experiences – if it exists – is a

subtle one, – a difference that can easily go unnoticed. The present speculation is meant

to illustrate the following insight: the difficulties of attaining adequate phenomenal

descriptions is often seriously underestimated. We should not be too surprised if even

16

experts in color vision and color phenomenology where to confuse two kinds of

experiences that are so closely related to one another that their difference is easy to be

overlooked – in particular in a case where one of the two kinds of experiences never

occurs in normal life.28 Therefore – or so we would like to claim – the possibility that the

experiences that occur under the relevant conditions in the laboratory are actually

misdescribed when described as experiences of reddish green is still open and does

have a certain plausibility. However, as we said, we do not claim that this possible error

has actually been committed. In the following discussion we will assume that no such

phenomenological mistake is involved in the description of the experiences at issue as

being of reddish green.

7. The philosophical challenge

But let us assume now that the strongest interpretation of the experimental results is

correct and that some subjects really had an experience of a new hue that is never

experienced under normal circumstances and that was phenomenally composed of red

and green for them then. It follows that red and green are not incompatible in the sense

of the following definition:

Definition: The hues H1 and H2 are incompatible iff (IC) It is impossible that a subject has an experience of a hue that is phenomenally composed of H1 and H2 for him or her then.

On the basis of one's own experiences of red and green it is however tempting to believe

that red and green are incompatible in this sense. Phenomenological reflection on the

particular character of red and green seems to reveal that these two hues cannot

possibly mix into a new hue in any possible experience. Phenomenological reflection

seems to support the particular modal claim about red and green at issue. However,

according to the present interpretation of the empirical results, we have to accept that

the modal claim has been shown to be false by empirical investigation. We thus seem to

be forced to conclude that phenomenological reflection is unreliable as a source of

modal knowledge about phenomenal structure. Given this consequence we have reason

to doubt other modal claims that are based on phenomenological reflection as well. We

17

have reason to doubt, for example, that violet is composed of red and blue (if the claim is

interpreted in the way we proposed).

In what follows we will argue that this doubt would be an overreaction. The possibility of

experiences of reddish greens or bluish yellows should not be taken to justify general

scepticism about phenomenologically based modal knowledge with respect to the

structure of color experience. To justify this claim we have to show that there is a

relevant disanalogy between the violet case and the reddish green case. We have to

show that the best justification available for the modal claim that no one can have an

experience of violet without thereby having an experience of a color that is then for that

subject phenomenally composed of red and blue is different in structure and stronger

than the best available argument for the modal claim that no one can ever have an

experience of a color that is for that subject on that occasion phenomenally composed of

red and green.

8. How to justify the modal claim about violet experiences

In order to see that the best justification for the claim we defend (the modal claim about

violet) is quite different in structure from the best justification of the modal claim that is

challenged by the empirical results at issue (the modal claim about reddish greens) we

have to propose first a reconstruction of the first justification. We have to give an answer

to the following questions: Why are we justified in believing that no subject can have an

experience of violet without thereby having an experience of a hue that is (for the subject

on that occasion) phenomenally composed of red and blue? What are the different steps

involved in the justification? What are the premises we need to accept if we take the

claim to be true? What are our reasons to accept these premises?

8.1. Phenomenal criteria of concept application

In preparation to our answer to these questions we have to say a few words about the

application of phenomenal concepts to one's own experiences. When learning the color

language we learn not only to apply color concepts to coloured objects but also to apply

color words to types of sensations or experiences. These two conceptual capacities are

18

of course closely related. For the present purposes we will focus on the second capacity.

A subject who has acquired the concept of a violet experience has a number of

capacities. The subject will consider an experience as falling under the concept just in

case it is a violet experience (the concept of course has vague boundaries), and the

subject will be able to imagine violet experiences, and the subject will be able to

consider the possibility that other subjects have this type of experience too. When

somebody has learned to apply the concept of violet experiences correctly to his or her

own experiences then he or she has learned to distinguish violet experiences from other

types of experiences by their phenomenal character. He or she has learned to

distinguish experiences according to a certain phenomenal criterion. In a sense the

person then has learned to apply a certain criterion.

But to talk of a 'phenomenal criterion' may be risky in this context. If we say that a

person applies a particular phenomenal criterion in her use of the concept of violet

sensations we wish to say – simply – that she recognizes a certain phenomenal

commonality (something all and only violet experiences have in common) and that she

applies the concept as a result of her recognition of this commonality. We presuppose

here that violet experiences do have something in common and that what they have in

common concerns their phenomenal character. In some cases the subject who applies a

certain phenomenal criterion in her use of a phenomenal concept does not have any

other concept that she could use to describe the criterion that she thereby applies. If for

example, a person should say what criterion she applies when applying the concept "red

experience" to her own experiences, then the person would not be able to say anything

informative about the criterion without again using the concept of a red experience. The

best she could probably say is that she calls an experience of her own a red experience

just in case it is an experience of a color with a strongly prevailing red component. So at

least the concept of a red component has to be used in the description of the

phenomenal criterion applied by a person who masters the concept of a red experience.

The same holds for the third person perspective. If A describes the phenomenal criterion

applied by B when using her concept of a red sensation, then A will have to talk of a

prevailing red component in the color experienced by B again. However, no danger of

any problematic circularity arises. There is no need to describe the phenomenal criterion

applied in applications of the concept of a red experience in any independent way.

19

In the violet case, however, we do have other phenomenal concepts that we can use to

describe the phenomenal criterion that we implicitly apply when using the phenomenal

concept of violet experiences. We can describe the criterion simply by saying that we

call an experience a violet experience just in case it is an experience of a color with the

hue violet. But we can also say the same in another way: we can say that we call an

experience a violet experience just in case it is an experience of a color that has for the

subject on that occasion a red component and a blue component where none of the two

prevails too much. (What would be "too much" is of course again vague.) So in the case

of violet we can describe the phenomenal criterion we apply when using the concept of

the type of experience at issue in terms of phenomenal composition.

It should be noted that, according to the realist and fallibilist view we defend, we may be

wrong about the phenomenal criterion we actually apply. Strictly speaking we express

only a hypothesis about the phenomenal criterion we actually apply when we say that we

call an experience an experience of violet just in case the condition just mentioned about

phenomenal composition is satisfied. The hypothesis might be wrong. A person may be

convinced that the phenomenal criterion for the application of the concept of a green

experience can be described as follows: the experience is an experience of a color that

is for the subject at issue phenomenally composed of blue and yellow. Yet, green is a

unique hue.29 So this is a false description of the phenomenal criterion we actually

apply. So although we are in a sense directly aware of the phenomenal criterion that we

apply we may nonetheless be wrong about the criterion (we may misconceptualize the

criterion). In the present case the misconception of the phenomenal criterion is due to a

false phenomenological description of one's own color experiences. A person who

believes that the phenomenal criterion for green experiences is the one just mentioned

will believe that she appropriately describes her own green experiences as experiences

of a binary color, a color that appears (to her on these occasions) to be composed of

blue and yellow.

How then can we rationally form a hypothesis about the phenomenal criterion that we

actually apply in the case of a given phenomenal concept of types of color experiences?

Our answer is that it is possible to rationally form a hypothesis of this kind on the basis of

phenomenological reflection on one's own color experiences. By carefully attending to

the subjective character of those experiences I learned to call violet, I am able to find out

20

what they have in common and to express what they have in common in terms of

phenomenal composition. Why is it allowed to base this general hypothesis on

observations of my own single case? The answer we propose is that we are entitled to

assume that we did learn to use the concept of violet experiences appropriately in the

following sense: we learned to apply the same phenomenal criterion that is applied by

other people as well when they use the phenomenal concept they express using the

term "violet experience". Of course, here again, there is room for a mistake. If I am

pseudonormal then what I call violet experiences is what other people call experiences

of bluish green (or greenish blue).30 I then have not learned to use the term "violet

experiences" correctly and I then associate the wrong phenomenal concept with the

phenomenal term.31 Under normal circumstances however - or so we assume - we are

justified in the belief that we express the same phenomenal concept when using the

term "violet experience" as other competent users of our language with normal color

vision. This is implies that we apply the same phenomenal criterion as they do when we

use the phenomenal concept we associate with the term at issue. But if we are so

entitled to believe that the phenomenal criterion is the same, then we are justified to

believe that a description of the phenomenal criterion we actually apply in our own case

is appropriate if and only if the description is also appropriate as a description of the

criterion applied by others when using the concept they associate with the relevant term.

Therefore we can arrive at justified claims about the phenomenal criterion applied in

uses of the concept of violet experiences in general by carefully attending to our own

single case.

8.2. Actual and counterfactual cases of color experiences

Until now we have been talking about the application of phenomenal concepts to actual

color experiences only. We have not yet been talking about possible color experiences

of other sentient beings in counterfactual circumstances. We have to turn to

counterfactual cases however since the claim we wish to justify is a modal claim about

all possible violet experiences. At this point it is important to see that phenomenal

concepts are special in the following respect: the criterion for their application to

counterfactual cases is the same as the criterion for their application to actual cases. If it

is correct, for example, that a person who masters the concept of violet experiences

applies the concept to all and only those experiences where the subject has an

21

experience of a color that is phenomenally composed of red and blue (with none of the

two components prevailing too much), then an experience in counterfactual

circumstances counts as an experience of this particular type just in case it fulfils this

specific phenomenal criterion.32 For a subject in counterfactual circumstances to fall

under the concept of having a violet experience it is necessary and sufficient that it has

an experience that fulfils precisely the specific phenomenal criterion actually applied to

their own color experiences by real people who have the concept. The reason is that to

fall under a particular phenomenal type is to fulfil a particular phenomenal criterion.

There is nothing else that is essential for having a given phenomenal property. There is

a particular aspect of the phenomenal character that is necessary and sufficient for

actual experiences to fall under the type "violet experiences". If some being in

counterfactual circumstances or in some other part of this universe has an experience

with this particular phenomenal aspect then the experience is ipso facto an experience

of violet. In other words: It is only this particular phenomenal aspect of the experience

that is essential for being a violet experience.33

Note that what we just claimed for the case of phenomenal concepts is not generally true

for all concepts. In many cases, as has been pointed out in the literature about natural

kind terms, the criterion of application in the real world (the way we pick out what falls

under the concept in the real world) may not coincide with the criterion of application in

our thoughts about counterfactual circumstances. Many philosophers admit that we pick

out the liquid water in the actual world by properties of water that are not essential to

water. In other words: the properties that a liquid must have to fall under our concept of

water in the real world are properties that a liquid in counterfactual circumstances might

lack although it is appropriately considered as being water. The parallel claim is false for

phenomenal concepts: if a certain phenomenal criterion has to be fulfilled by an actual

experience to fall under our concept of a violet experience, then no experience in

counterfactual circumstances that fails to satisfy this criterion falls under the concept of a

violet experience.34 It follows that any claim about the phenomenal criterion we apply to

actual experiences when using a particular phenomenal concept is also true as a claim

about the criterion for counterfactual cases. Therefore the following conditional holds: If

CR is the phenomenal criterion for application of the phenomenal concept C to actual

experiences then nothing in counterfactual circumstances falls under C unless it fulfils

CR. For the case of violet the conditional reads as follows: If it is true that the

22

phenomenal criterion for correct applications of the concept of violet experiences is that

the experience at issue is of a color that is for the person at issue on the given occasion

phenomenally composed of red and blue (where none of the two components prevails

too much), then a being in counterfactual circumstances has an experience of violet just

in case it has an experience of a color that is for it on that occasion phenomenally

composed of red and blue (where none of the two components prevails too much). This

is of course the result we wanted. We have justified the step from a claim about a

phenomenal criterion as it is applied in actual cases to the modal claim at issue about all

possible experiences.

8.3. Summary of the proposed reconstruction

In our thoughts we use phenomenal concepts to attribute phenomenal properties to

other people or to other sentient beings. We use the phenomenal concept of having a

violet experience to attribute the phenomenal property of having a violet experience. In

many cases we can use language to express what we think. We use the predicate "has

a violet experience" in order to attribute the property of having a violet experience under

its phenomenal conceptualization. This does not mean that a person who does not have

the phenomenal concept of a violet experience cannot use the corresponding

phenomenal term to attribute a certain phenomenal property. A blind person may lack

the phenomenal concept of a violet experience and yet use the term "violet experience"

with the intention to ascribe a particular phenomenal property to another person. Yet,

there is a close relationship between the phenomenal concept of having violet

experiences and the corresponding phenomenal predicate "has a violet experience". We

can describe the relation as follows: If a person thinks that some X has a violet

experience and thereby uses the phenomenal concept of violet experiences, then that

person would use the sentence "X has a violet experience" correctly if she used it to

express that thought. To have a name for this relation between phenomenal concepts

and phenomenal predicates we will say that the concept is associated with the predicate

and we will thereby presuppose something like the following definition:

Definition: A phenomenal concept C is the concept associated with the phenomenal predicate PP just in case the following condition is fulfilled: If a person A has the thought that X has the property P and conceptualizes the

23

property P via the phenomenal concept C and if A expresses this thought saying "X has PP", then A uses PP correctly.

According to this definition the phenomenal concept associated to a particular

phenomenal term is the relevant concept involved in a thought that can be correctly

expressed using the phenomenal term at issue. With this terminological preliminary in

place we can reformulate and summarize the argument developed in the two preceding

sections.

The justification of the modal claim that no sentient being can have an experience of

violet without thereby having an experience of a color that is for it then phenomenally

composed of red and blue can now be formulated in the form of an argument. (Since

phenomenological descriptions of one's own experiences play a role we will formulate

the argument in the first person).

Premise 1 (hypothesis about my own phenomenal criterion): The phenomenal criterion for my application of my phenomenal concept to my own experiences can be formulated as follows: I accept something as being a violet experience just in case it is an experience of a color that is for me then phenomenally composed of red and blue (where none of the two prevails too much).

Premise 1 expresses a phenomenological description. A person who accepts premise 1

accepts a certain phenomenological description of all those experiences he or she

subsumes under the phenomenal concept of violet experiences.

Premise 2 (assumption of correct language use): My phenomenal concept of having violet experiences is the concept associated with the predicate "has violet experiences".

This premise is meant to imply:

Consequence 1: My phenomenal criterion for the application of my phenomenal

concept of violet experiences coincides with the phenomenal criterion for the concept

associated with the term "has a violet experience".

Premise 1 and consequence 1 imply:

24

Consequence 2 (correct application of the corresponding term): An actual experience falls under the phenomenal concept associated with the predicate "has a violet experience" iff the subject at issue has an experience of a color that is phenomenally composed of red and blue for it then (where none of the two components prevails too much). Premise 3 (counterfactual extension of phenomenal concepts): In the case of phenomenal concepts the phenomenal criterion of application to actual experiences coincides with the phenomenal criterion of application to counterfactual cases. Consequence 2 and premise 3 imply:

Consequence 3 (the modal claim formulated in terms of concept application): No experience of a being in counterfactual circumstances falls under the phenomenal concept of having violet experiences unless it has an experience of a color that is for it then phenomenally composed of red and blue.35

We arrived, almost, at the modal claim we wanted to justify. We still have to get rid of

talk of concept application since we wish to talk of the type of experience itself. To do

this we simply have to assume that what falls under the phenomenal concept of violet

experiences in counterfactual circumstances is a violet experience.

Premise 4 (link between concept application and the exemplification of the corresponding property in counterfactual circumstances): A being (in counterfactual circumstances) falls under the concept of having a violet experience iff it has a violet experience.

We now can conclude (using premise 4 and consequence 3):

Consequence 5 (the modal claim about violet experiences): Necessarily, if a being has an experience of violet it has an experience of a color that is for it then phenomenally composed of red and blue.36

We normally are in an epistemic position where it is justified to assume the first two

premises. Premise 3 is a general conceptual truth concerning phenomenal concepts and

premise 4 is trivial. So the modal claim at issue has been shown to be justified (for every

person in a normal epistemic situation). Of course this does not exclude the possibility of

error. We cannot be certain that our phenomenological description of our own

experiences is correct (we might be wrong about the phenomenal criterion we apply,

premise 1 may be false). Premise 2 might be false too. No singular person can exclude

that he or she is a deviant color perceiver and that her or she systematically

25

misunderstands phenomenal terms for that reason. However, there normally is no good

reason for this doubt. In the normal case it is possible to form rationally justified belief

about modal claims concerning phenomenal composition in color experiences of a given

type. A person can (a) form a hypothesis about the relevant phenomenal criterion for her

own application of the phenomenal concept of the type of color experience at issue. He

or she can then (b) assume that this criterion coincides with the phenomenal criterion of

application for the phenomenal concept associated with the corresponding predicate and

(c) conclude that this criterion coincides with the criterion of application for that concept

in counterfactual cases. Step (a) involves a phenomenological analysis of one's own

experiences of the relevant type, step (b) presupposes conformity of one's own

understanding of a phenomenal term with the understanding of competent speakers of

the language and (c) uses a conceptual fact about phenomenal concepts.37

9. How to justify the modal claim about reddish greens

Does the case of reddish greens undermine the modal claim about violet? The possibility

of experiencing reddish greens would undermine the modal claim we just defended if the

best justification for the intuitively plausible modal claim that it is impossible to

experience a reddish green were relevantly similar to the justification we gave for the

modal claim about violet. If that justification looks very much like the one we gave for the

modal claim about violet then (given the empirical results reported above) there would

be reason to doubt the argument that led to the modal claim about violet as well. To see

if this is the case we have to find a justification for the modal thesis about reddish green

that is plausibly the best candidate for a rational reconstruction of the reasons we have

for believing in that claim.

It may appear as though we are able to judge that red and green cannot possibly

combine to yield a new hue just by attending to the particular quality of red and green.

Given their intrinsic quality (somebody might say) it is evident that they cannot be

present in the sense of phenomenal composition in the experience of one hue. If this is

the way to justify our belief in the modal claim at issue about reddish greens, then the

way we arrive at the modal thesis is quite direct. According to this picture a person who

has the phenomenal concept of having red experiences and the phenomenal concept of

26

having green experiences and who understands the notion of phenomenal composition

can see (intuit) that there is no possible hue such that a person could have an

experience of that hue and thereby have an experience of a color that is phenomenally

composed of red and green. If this is the correct view then the modal claim about

reddish greens is justified by what has been called rational intuition: understanding the

notions involved suffices for the capacity to have the modal insight at issue.38 Let us call

this view about the best justification of the modal claim about reddish green the rational

intuition view with respect to reddish green.

According to the rational intuition view the best justification of the modal claim about

reddish green is quite different in structure from the best justification of the modal claim

about violet. According to that view understanding the notions involved makes the modal

claim compelling or intuitively obvious. A person who understands the concepts involved

cannot but intuit that red and green cannot combine in experience into a new hue.

According to the justification proposed in section 8 the situation is quite different in the

case of the modal claim about violet. If the rational intuition view about reddish green is

right then there are no steps in the reddish green case analogous to those present in the

justification of the modal claim about violet. No premise about the phenomenal criterion

of application of phenomenal concepts plays any significant role. There is thus no

premise needed that would be analogous to the first premise of the above argument.

The first premise in that argument is the result of phenomenal reflection on particular

color experiences belonging to the kind in question. It is clear that the case for the modal

claim about violet cannot start with a phenomenological description of experiences of the

type we call green experiences or the type we call red experiences. That green cannot

combine in the way at issue with red is nothing that can be part of a phenomenological

description of a green experience (and the analogous claim holds for red). By carefully

attending to violet experiences we are able to discern a reddish component. If we

discern the reddish component, then we have been able to isolate (in reflection) a

particular aspect of the phenomenal character of experiences of the relevant kind. By

contrast, the impossibility of green to combine with red is not an aspect of the subjective

quality of the experience of green in itself. Therefore, phenomenological reflection in the

strict sense of finding out what phenomenal aspect experiences of a particular kind have

in common clearly is no possible starting point for any justification of the modal claim

about reddish greens.

27

Let us turn to the second premise. A person who asserts the modal claim at issue

wishes to say something about red and green experiences. Red and green experiences

are those experiences that are attributed in thought using the phenomenal concepts

associated with the term "red experiences" and "green experiences". If a person has

reason to doubt that he or she understands the terms "red experiences" and "green

experiences" correctly then she cannot use her conceptually based intuition to judge the

truth of the modal claim (that is of course formulated using the linguistic terms "red

experience" and "green experience"). Therefore, on the rational intuition view, a

justification of the modal claim has to include an assumption analogous to premise 2 in

the above argument. So here we find a parallel. The parallel, however, is a trivial one.

In the argument for the modal claim about violet the conceptual claim about phenomenal

concepts, premise 3, plays a prominent role. There is no such step involved in the

justification of the modal claim about reddish greens if the rational intuition view is the

one to adopt. We do not, in that case, first form a hypothesis about phenomenal criteria

of concept application to actual cases and then generalize to counterfactual cases (using

premise 3), since there is no hypothesis about phenomenal application to actual cases

involved in the first place. Rather, according to the rational intuition view, we go on

directly to a claim about counterfactual cases by modal intuition based on concept

understanding.

We conclude that – according to the rational intuition view and presupposing the above

justification concerning the violet case as the best justification available – there is no

structural similarity between the two justifications. The two justifications are

fundamentally different. Therefore, the empirical results that undermine the impossibility

claim about reddish greens cannot undermine the necessity claim about violet.

To arrive at this result we assumed, however, that the rational intuition view is the one to

accept. But there might be a better justification of the modal claim about reddish green

that is also more similar in structure to the argument given for the modal claim about

violet. If this were true, then the empirical results about reddish greens would undermine

the necessity claim about violet after all. How would a justification of this kind look like?

28

A justification of this kind might start with the observation that (a) by carefully attending

to our own color experiences we can find out that we never have an experience of a hue

that is phenomenally composed for us then of green and red and (b) that whenever we

try to imagine an experience of a hue that is phenomenally composed of red and green

we do not succeed in fulfilling that imaginative task. The first observation, however, is a

bad starting point. There simply is no rational way from the observation that we do not

ever have an experience of a color that is for us then phenomenally composed in a

particular way to the belief that necessarily no subject can ever have an experience of a

color that is for him or her then phenomenally composed in the way at issue. More to the

point, no premise analogous to premise 3 can help to get from the observation about

actual cases to the claim about counterfactual cases. Premise 3 can help to make that

step only if we have a phenomenological description of concrete actual cases as a

starting point. But the observation mentioned in (a) does not provide a

phenomenological description of actual cases. So maybe our incapacity mentioned in (b)

is a better starting point. But how can we rationally conclude from the mere fact that we

are unable to imagine a reddish green to the modal claim that it is impossible to

experience a reddish green? Maybe somebody can come up with a convincing

justification of this step. If so, however, it certainly will not have the structure of the

argument for the modal claim about violet. Again, no premise similar to premise 3 can

possibly be used in an argument defending the step from our incapacity of imagination to

the relevant impossibility claim. The reason is the same as before. Premise 3 can help if

we can start with a phenomenological description of actual experiences that belong to a

particular kind. But the observed incapacity mentioned in (b) does not provide a

phenomenological description of actual cases.

Our main point here is not to attack the modal claim about reddish green or to show that

it is irrational to hold that claim. We rather would like to be neutral on this point for the

moment. What the discussion should show is this: there is a fundamental disanalogy

between the best way to justify the modal claim about violet and the best way to justify

the modal claim about reddish green. Therefore, it would be a mistake to think that the

empirical results about reddish greens can challenge the modal claim about violet (and

other similar claims about phenomenal composition).

29

10. Objections

First objection

Simplifying a bit we may say that the above argument implies the following claim: A

person who forms (on the basis of carefully attending to her own color experiences) the

belief that all her violet experiences are experiences of a color that is for him or her then

phenomenally composed of red and blue has reason to believe that nothing can count

as a violet experience unless it has that particular property. But now imagine the case of

a person, Rosa, who for some contingent reason never sees any yellowish red but only

pure red or bluish red. When Rosa carefully attends to her own color experiences she

will observe that all her red experiences have the following property: they are either an

experience of a unique hue or they contain a bluish component. Can she rationally

conclude that all experiences of red necessarily have that particular property? Intuitively

it seems that she cannot and obviously the conclusion would be false. But the case

seems to be perfectly parallel to the violet case. So something must have gone wrong.

Answering the first objection

The objection rests on a misunderstanding of the first step in the argument. To form a

hypothesis about the phenomenal criterion associated with a specific phenomenal

concept is not simply to become aware of a particular commonality of all those

experiences that fall under the type at issue and that the epistemic subject had in her

own past. Suppose that a person, we will call her Violetta, never sees any violet except

for a few rare occasions where she is presented with violet stars of different shades

painted on a white surface. Suppose somebody tries to teach the term "violet

experience" to Violetta. There are two possibilities: either Violetta understands the

explanation and forms the phenomenal concept associated with the term violet or she

does not. If she does understand the explanation then she would apply that concept to

every violet experience. In this case, if somebody were to present an object to Violetta

that looks violet and circular, then Violetta would consider it to be one further case of a

violet experience. If this first possibility is realized, Violetta would be aware of the fact

that she herself actually never had (and might not ever have in the future) any violet

experience that is not at the same time an experience of something with the shape of a

star. However, Violetta would also be aware of the fact that the shape is irrelevant for

the application of the phenomenal concept that she has been taught. The example

30

illustrates that we have to distinguish – as said before – between the observation of a

mere commonality shared by all actual experiences that the epistemic subject ever had

in her own life and the formation of a hypothesis about the adequate phenomenal

criterion associated with the concept at issue or with the linguistic term at issue. If

Violetta misunderstood what people meant when using the terms “violet” and “violet

experience” at her presence then she will have formed a different phenomenal concept

(non-identical to the concept of violet experiences) and she will incorrectly believe that

this is the concept associated with the term at issue. Given her epistemic situation she

may not have formed the concept of violet experiences but the phenomenal concept of

the experience of something that looks both violet and star-formed. She would not be

ready to apply that concept to the experience of a violet elephant. But Violetta need not

make this mistake. She may well be aware of the fact that being star-formed is not part

of the phenomenal criterion for the application of the phenomenal concept associated

with the term “violet experience” although she never saw anything that looked violet to

her and not star-formed.

Let us now turn to the case of Rosa. In Rosa's case we have to make the same

distinction. We have to distinguish the following two possibilities: (a) Rosa has formed

the concept of a red experience and she believes correctly that this is the concept

associated with the term "red experience". (b) Rosa has formed the concept of an

experience of red or bluish red and she believes incorrectly that this is the concept

associated with the term "red experience". If the possibility (a) is realized, then Rosa

would subsume an experience of her own under her relevant phenomenal concept if and

only if it is an experience of a color for her that contains a prevailing reddish component.

The concept Rosa associates with the term “red experience” is the phenomenal concept

of a red experience only if she would apply the concept in this way. This possibility can

be subdivided into two further possibilities:

(aa) When reflecting upon her own red experiences Rosa is able to find out that the

criterion she applies when classifying them as red experiences is the one mentioned

before. She then will accept the premise for red experiences that corresponds to

premise 1 for violet experiences despite her special and slightly impoverished

experiential background. If this is the case under consideration, then Rosa poses no

31

problem for the argument. Her special situation will entail no epistemic mistake and she

will rationally arrive at the correct modal conclusion.

(ab) When reflecting upon her own red experiences Rosa will come up with a false

hypothesis about the phenomenal criterion that she actually applies. Although she would

classify slightly yellowish reds as experiences of red if she ever had this kind of color

experience she is not aware of this fact. In this case Rosa will introduce a false first

premise into her argument:

Premise 1 (hypothesis about my own phenomenal criterion): The phenomenal criterion for my application of my phenomenal concept to my own experiences can be formulated as follows: I accept something as being a red experience just in case it is an experience of a color that is for me then either only reddish or phenomenally composed of red and blue (were the red component prevails).

The premise corresponding to premise 1 in the above argument in her corresponding

argument would be false. This possibility however provides no argument against the

proposed justification. False premises may lead to false conclusions. Of course a person

may construct an argument of the above structure and yet arrive at a false conclusion if

she introduces at one step a false premise.

Let us now return to the possibility mentioned above under (b). Rosa then has not

acquired the concept of red experiences on the basis of her red experiences but a

different and more restricted concept that only includes experiences of bluish reds and

pure reds. In this case Rosa may arrive at the wrong conclusion mentioned in the

objection. But this will be due to another mistake: she will believe, incorrectly, that her

phenomenal concept at issue is the concept associated with the term "red experiences".

Again, Rosa's case poses no problem. She arrives at a false conclusion although she

develops a valid argument simply because she introduces at one point a false empirical

assumption. The premise corresponding to premise 2 in her argument would be false.39

Second objection40

It is possible to justify the modal claim about reddish green in a way that parallels quite

perfectly the justification of the relevant modal claim about violet. The basic idea is to

use the phenomenal concept of color experiences in general and to assume that it is

part of this concept that the hues involved are the four hues known to us and some (but

32

not all) phenomenal combinations. According to this argument an experience of reddish

green simply does not count as a color experience (according to our phenomenal

concept of color experiences). This parallel argument has the following premises (where

premise 1*, premise 2* and premise 4* are premises about the phenomenal concept of

‘having a color experience’ analogous to the premises 1, 2 and 4 about the phenomenal

concept of violet experiences and premise 3 remains the same).

Premise 1*: The phenomenal criterion for my application of the phenomenal concept of having a color experience to my own experiences can be formulated as follows: I accept something as being a case of having a color experience if and only if it is an experience of a color that is for me then either unique red, blue, green or yellow or is phenomenally composed of red and blue or blue and green or green and yellow or yellow and red. Premise 2*: My phenomenal concept of having a color experience is the concept associated with the predicate 'has a color experience'. Premise 3: In the case of phenomenal concepts, the phenomenal criterion of application to actual experience coincides with the phenomenal criterion of application to counterfactual cases. Premise 4*: A being (in counterfactual circumstances) falls under the concept of having a color experience iff it has a color experience. Just as in the case of the modal claim about violet these premises lead to the following

result:

Consequence: Necessarily, if a being has a color experience it has an experience of a color that is not for it then phenomenally composed of red and green.

There is a simple and direct way to answer this objection. The empirical data support the

claim that it is possible to have an experience of something that appears reddish and

greenish in the sense of phenomenal composition. These data cannot settle the issue

about whether these experiences are appropriately called color experiences. Therefore

the stronger claim that there are possible color experiences where something appears

reddish green to the person at issue is not supported by the data. It follows that the

existence of an argument for this stronger claim that is analogous to the proposed

argument for the modal claim about violet is no danger for the thesis about violet since

there is no compelling reason – based on the empirical data at issue – for the view that

the analogous argument for the stronger claim must be deficient.

33

Given the dialectical situation just mentioned we do not need to refute the parallel

argument in order to defend the proposed justification for the modal claim about violet

against any danger caused by the empirical data about reddish green. But we cannot be

quite happy with this response. Even if the empirical data cited in this paper do not in

themselves support the stronger claim about possible color experiences our argument

should not depend on the dubious thesis that reddish green experiences could not be

counted as color experiences. But, unfortunately, the argument does depend on this

assumption if the parallel argument were to go through: if philosophical reflection

together with the empirical data can convincingly support the view that there are color

experiences of reddish green, then the existence of an argument similar in structure to

ours showing the contrary still represents a problem for the modal claim about violet. In

defence of the proposed justification of the modal claim about violet we therefore have to

say what is wrong with the parallel argument and in what way it is relevantly different

from the one we endorse.

To do this it will be helpful to introduce – just for the present purposes - two senses of

‘color experiences’, a broader sense and a strict sense. According to the strict sense an

experience can count as a color experience only of its corresponding hue is one of those

hues human normal perceivers are acquainted with. We will indicate this use of “color

experience” by adding the subscript “1”. The possibility of color1 experiences of ‘alien’

hues or ‘novel hues’ are excluded by stipulation. In the broader sense of color

experiences (indicated by the subscript “2”) someone may well have an experience of a

color with a hue not known to normal human beings. Let us assume, for the sake of the

argument, that it makes sense to talk of color2 experiences. Although we cannot imagine

having color experiences of alien or novel hues we have a sufficiently clear

understanding of the commonality of all those experiences that are appropriately to be

summarized under “color2 experiences”. If pressed to say what this commonality

consists in we would have to describe phenomenological facts about colour experience

that are not plausibly restricted to experiences of our ‘human’ hues. This certainly is not

a trivial task but also it is not trivial that it cannot be accomplished in a satisfying way.

Given this distinction the above parallel argument has two interpretations. According to

the first interpretation the argument is about color1 experiences. In this case the

consequence is trivially true and so no problem arises for the argument we wish to

34

defend. According to the second interpretation the argument is supposed to show that

we are simply unable to generalize the concept of color experiences so that it covers

experiences of novel and alien hues. But, when understood in this way, the argument is

unconvincing. It is quite clearly impossible to settle the issue about whether talk of color2

experiences makes sense by simply assuming that we actually use the term “colour

experiences” in the way described by premise 1. There are two ways to make this point.

We can either assume – quite plausibly – that our actual uses of the concept of colour

experiences underdetermines the question of whether we apply the phenomenal

criterion associated with the concept of color1 experiences or the phenomenal criterion

associated with the concept of color2 experiences. But suppose – for the sake of

argument – that the issue is not underdetermined and that premise 1 is correct. This

observation of how we actually use the term does not, however, settle the question of

whether we can develop a general concept of colr experience, a concept of colour2

experiences that does allow for novel and alien hues. Therefore, in its second possible

interpretation the argument does not go through.

It remains to be said what distinguishes the parallel argument in its second interpretation

from the argument leading to the modal claim about violet that we endorse.

Corresponding to the two ways to criticize the parallel argument just mentioned there are

two ways to describe the difference. If we assume – quite plausibly – that our actual use

of the term “color experience” underdetermines the issue of which of the two

phenomenal criteria we actually use, then the relevant difference lies in the fact that

there is no such underdetermination in the case of the phenomenal criterion of the

concept of violet experiences. If, on the other hand, we assume for the sake of argument

that the issue about which if the two phenomenal criteria we actually apply in uses of the

term “color experience” is not underdetermined (and determined in the way stated in

premise 1), then we have to say that the difference lies in the fact that the real issue – in

the case of the concept of color experience – concerns the question of whether we have

or can develop a generalized concept of color experience. Prima facie at least the

concept of color2 experiences seems to make sense and this is precisely the reason why

the consequence of the argument will be controversial. But there is no parallel

generalized concept of violet experience in play. It makes sense to speculate if we have

or can develop a concept of color experiences on the basis of phenomenological

commonalities between all color experiences that however are independent of the

35

specific hue. The parallel speculation does not make sense in the case of the concept of

violet experience. There is no prima facie plausibility to the claim that we can develop a

generalized notion of violet experience where the hue experienced is not constitutive for

membership in the corresponding kind.

Third Objection

Despite the differences between the two justifications there is a common element that

makes them sufficiently similar for the empirical results about reddish green to

undermine the modal claim about violet as well. Let us assume that the rational intuition

view is rejected and that we are left with the imagination argument as the best argument

for the modal claim about reddish green. Then the common element is the role of

imagination in both arguments. Obviously in the second argument (for the impossibility

claim concerning reddish green) imagination plays a crucial role. From the fact that we

cannot imagine an experience of a color that is phenomenally composed of red and

green we conclude that no experience of this kind is possible. In the first argument, as it

has been presented, no imaginative act seems to be important at any point. A closer

look upon the argument, however, reveals that imagination does play an important role

in the justification at issue. We see this if we have a closer look at the way we arrive at

premise 1. How can I find out that my phenomenal criterion for applying my concept of a

violet experience is appropriately described in the way that has been proposed? It is not

sufficient to consider actual cases of experiences that I had in the past. I have to form an

opinion about how I would react to imagined cases as well. (This is what we learned

from the answer to the first objection. Rosa can form an appropriate hypothesis about

the criterion she applies only by considering new cases that she never has been

confronted with.) So we do have to rely on our capacity to imagine color experiences. To

form a belief that we can state as premise 1, we have to be sure that (a) if we were

confronted with a color that is not phenomenally composed in the relevant way, than we

would not consider it as falling under the concept and that (b) if we were confronted with

a color that is composed in the right way, then we would subsume it under the concept.

To test (a) and (b) by imagination we cannot do anything else but this: we try to imagine

an experience of a color that is phenomenally composed in the right way and that we

would not count as falling under the concept and we try to imagine an experience of a

color that is composed in the right way but that we would not consider as falling under

the concept. If we fail in both imaginative acts, then we accept premise 1. This is — it

36

seems — the best and only way to justify premise 1. But if so, then imagination and in

particular the incapacity to imagine an experience with certain properties, is crucial for

the first argument just like it is crucial for the second and so the analogy is re-

established.

Answer to the third objection

We must admit that imagination plays a role in the justification of premise 1 in the way

described in the objection. However, this role of imagination is quite different from the

role of imagination in the justification of the modal claim about reddish green. In the

justification of premise 1 of the argument for the modal claim about violet imagination is

necessary to test one's own phenomenal criteria of concept application. If we do not

succeed in imagining a hue experience that fulfils a certain criterion but that we would

not count as falling under a particular concept and vice versa, then we have reason to

believe that we found the criterion that we actually apply. The test is not infallible of

course. We might fail to imagine a particular experience that would falsify our hypothesis

about our own application criterion associated with a given phenomenal concept, but still

the imagination test is a legitimate test of a given hypothesis about our own phenomenal

criterion of application. Contrary to this, imagination is not used to test phenomenal

criteria of application in the argument for the modal claim about reddish green. In this

case we try to imagine an experience that fulfils a particular phenomenological

description (the description of being the experience of a hue that is phenomenally

composed of red and green). If we find ourselves incapable to fulfil this imaginative act,

then we take this to be an argument for the impossibility of an experience that fulfils the

description. So the role of imagination in the two cases is radically different in two ways:

(a) In the first case we try to imagine a color that fulfils a certain phenomenological

description concerning phenomenal composition and if we succeed we go on to test

whether this would count as falling under the phenomenal concept at issue. We can test

our hypothesis about the phenomenal criterion of application only if we succeed in

imagining an experience that fulfils the relevant phenomenological description. Contrary

to this, in the second case, we try to imagine an experience that fulfils a particular

phenomenal description concerning its phenomenal composition and take our failure to

do so as support for the modal claim. (b) In the first case, the imagined experience is

used to test our phenomenal application criteria of a given phenomenal concept. This is

clearly a legitimate test that is not in need of any special additional argument. In the

37

second case, the incapacity to imagine an experience that fulfils a particular phenomenal

description is taken to be an argument for the impossibility of an experience of this kind.

This is clearly a step that does need an additional argument. While it is unproblematic to

use imagined experiences to test one's own application criteria it is highly problematic to

take one's incapacity to imagine something as an argument for its impossibility. No such

step from our incapacity to imagine something to a claim about metaphysical

impossibility is at issue in the argument concerning violet.

Fourth objection

The argument for the modal claim about violet is correct but the consequence depends

on a terminological (or conceptual) decision. We decide to call an experience a violet

experience just in case it is an experience of a color that is phenomenally composed of

red and blue (where none of the two components prevails too much). An experience has

to fulfil this condition in order to fall under the concept by stipulation. We could however

just as well stipulate that nothing falls under the concept of being an experience of green

unless it is either an experience of pure green or an experience of green with a slightly

yellowish or slightly bluish component. Given this stipulation, an experience with a

reddish component does not count as an experience of green. In this way we exclude

the possibility of experiencing reddish greens by stipulation. We could do the same for

the concept of a red experience. An experience with a greenish component would not

count as a red experience and so again – by stipulation – experiences of greenish red

are excluded. We thus arrive at the modal claim that it is impossible to have experiences

of reddish green or greenish red but we arrive at this conclusion by mere stipulation. It is

clear that we cannot decide any interesting substantial issue about what is essential for

certain kinds of experiences merely by stipulation. So the modal claim at issue really

turns out to be quite uninteresting and it is not open to any empirical rejection. This

reasoning about the reddish green case has consequences for the modal claim about

violet as well. What seems to be a substantial claim about metaphysical necessity really

is nothing but a trivial consequence of our decision to use the concept ‘violet experience’

in a particular way.

Answer to the fourth objection

According to this objection both modal claims are the trivial result of our decision to use

the concepts involved in a particular way. Let us call this view the stipulative view (about

38

the modal claim under consideration). The stipulative view is however false for both

cases. We will first discuss the stipulative view about the impossibility claim concerning

reddish greens.

According to the fourth objection we could take the following decision: If a person had an

experience of a color that is phenomenally composed for her then of red and green, then

we will neither count this experience as being an experience of a reddish green nor

count this experience as being an experience of greenish red. We simply decide that

color experiences do not count as experiences of red if they contain a greenish

component and do not count as green if they contain a reddish component. We thus

arrive at the modal claim at issue in a trivial way. There are two points to be made about

this proposal. First, the question was whether it is possible to have an experience that

has both a red and a green component. We cannot exclude this possibility by deciding

that we would not call an experience of this particular kind an experience of reddish

green or greenish red. In formulating his stipulation the proponent of the present

objection uses the term “red” and the term “green” to refer to a specific potential

phenomenal component of a color as it is perceived by a subject. But once the reference

to these components is assumed as fixed we can ask whether or not it is possible to

have an experience of a color that contains for the person at the relevant moment both a

reddish and a greenish component. According to the modal claim the answer to this

question is “no”. The proposed stipulation however is silent about the answer to this

question. The proposed stipulation excludes that we call an experience of a color an

experience of reddish green or greenish red but it does not exclude that a perceived

color might be phenomenally composed of red and green. So, contrary to the above

objection the modal claim we are interested in is no trivial consequence of the proposed

stipulation.

A second point to stress is that the proposed stipulation is unacceptable given the use of

the rest of our phenomenal terms. We call an experience an experience of a reddish

blue just in case it is an experience of a color that is phenomenally composed of red and

blue (where the blue component prevails). To decide that an experience that is in fact an

experience of a color that is for the subject on that occasion phenomenally composed of

red and green (with e.g. a prevailing green component) should not be called an

experience of reddish green would be a highly misleading deviation. From a more

39

abstract point of view we may say that a person who finds the proposal plausible might

not have seen that there are two different uses of the term “red” in descriptions of

experiences. We use the term “red” in “has a red experience” to express the property of

having an experience of a color that comprises many different shades (from almost

orange to almost violet). To have a red experience is to have an experience of a color

that has a sufficiently prevailing red component where a blue or a yellow component can

be present as well. The second use of “red” in the context of the description of

experiences is its use to refer to a particular phenomenal component in a color as it is

perceived by a subject. Once the meaning of our color component talk is fixed (once we

know what specific potential components we refer to using the words “red” and “green”)

the modal question at issue can be formulated and its answer cannot be found by any

decision about how to use the broad concept of having an experience of red in the sense

that comprises a great range of color shade experiences. We conclude that the

stipulative view about the impossibility claim concerning reddish greens is false.

The reconstruction we have proposed for the best justification of the necessity claim

concerning violet might invite the stipulative view about this modal thesis. After all the

argument uses assumptions about the use of the phenomenal concept of violet in two

places: in premises 1, where a claim about the phenomenal criterion of application is at

issue and in premise 3 where it is assumed that in the case of phenomenal concepts

there is no difference between the criterion of application to actual or to counterfactual

cases. However, the stipulative view misinterprets the status of premise 1. Premise 1 is

not the result of an arbitrary decision. It is not open to us to take a decision about the

phenomenal criterion associated with the use of the phenomenal concept of violet

experiences. Rather, we already have the phenomenal concept of violet experiences

and then try to find a description of the phenomenal criterion we actually apply by trying

to become aware of and to conceptualize the phenomenal aspect shared by those and

only those experiences that we are (or would be) ready to subsume under the concept.

Also, the stipulative view misinterprets the way we arrive at the phenomenal criterion for

the application of the concept to counterfactual cases. Here again we do not simply

decide that we would not count an experience as being an experience of violet unless it

is an experience of a color that is phenomenally composed for the subject at issue on

the relevant occasion of red and blue. Rather, we are forced to accept this constraint on

the application of the concept to counterfactual cases if our phenomenological

40

hypothesis in premise 1 is correct. But premise 1 is a phenomenological hypothesis that

might be wrong. So we are not free to decide about how we apply the concept to

counterfactual cases. How we have to apply it to counterfactual cases depends on how

we do apply it to actual cases and the latter is already fixed. This dependence is of

course based on the conceptual insight expressed in premise 3. Here again, somebody

might see room for an arbitrary decision. Couldn’t we decide to use our phenomenal

concepts in a way that makes – contrary to premise 3 – other aspects of the experience

essential for having an experience of the relevant kind? The answer is again no. Our

phenomenal concept of having violet experiences is a phenomenal concept and this

implies that nothing but the particular phenomenal ‘feel’ is relevant to for the

categorization we make using the concept. It is not up to us to decide what we consider

as essential. In introducing a concept of “having a violet experience” where it is not the

phenomenal criterion that is relevant for the application to counterfactual cases but

something else (say the way the experience is caused or the way it is realized in the

brain) would be to introduce a different concept and thereby to change the topic.

11. Concluding Remarks

We have argued that the modal claim about reddish greens has been challenged by the

empirical results at issue but that the modal claim about violet remains intact. Could

there be other empirical results that could give us reason to reject the claim that no

subject can have an experience of violet without thereby having an experience of red

and blue? Could it be that somebody constructs an experiment tomorrow where under

unusual circumstances people seriously and correctly claim to have seen a violet that

did look neither reddish nor bluish to them then but e.g. looked like a unique hue or

looked like a color phenomenally composed of red and yellow? According to the view we

have proposed an empirical rejection of the modal claim about violet is not a priori

excluded. We might be wrong in our assumption about the phenomenal criterion we

actually apply when using the phenomenal concept of violet experiences. Premise 1 is

open to empirical counterarguments. However, if we all associate the same phenomenal

concept with the term “has a violet experience” and if the phenomenal criterion for

applications of that concept is the one we mentioned then the possibility of empirical

41

rejection of the modal claim is excluded. An empirical argument against the modal claim

would have to be an argument against one of the preconditions just mentioned.

These reflections may raise the issue about the epistemic status of modal claims about

phenomenal structure of color experiences. In the case of reddish greens, can we arrive

at the modal claim a priori? If the rational intuition view is correct, then the answer

seems to be “yes”. According to that view to have the relevant concepts is sufficient to

rationally intuit the modal claim. But note that the modal claim is nonetheless open to

empirical rejection. Also, acquiring the concepts involved requires phenomenological

reflection on one’s own color experiences. Something similar applies to the modal claim

about violet. Whoever has the relevant concepts is in principle able to see the truth of

premise 1. Premise 1 therefore seems to be a claim that we can know in a sense a

priori. However, the claim involves an empirical thesis about the actual criteria of

application of one’s own phenomenal concept that is open to empirical falsification. On a

closer look it might well turn out that phenomenologically justified modal claims about

phenomenal composition (like the one about violet) do not fit well into the traditional

distinction between a priori and a posteriori knowledge. But we have to leave these

related epistemological issues to another occasion.

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35. Thompson, E. (1992). “Novel colours.” Philosophical Studies, 68(3), 321-349.

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1 We would like to express our gratefulness to the Swiss National Science Foundation, who funded the research of which this paper is part. The other members of our research group, Vivian Mizrahi, Dinah Gross and Achill Schnetzer contributed a lot by their objections and proposals to the development of the ideas presented in the paper. We are grateful to Fiona MacPherson who sent us extensive and very elaborate critical comments that motivated a number of changes and additions to the text. We also got very helpful written comments by Larry Hardin, Peter Ross and Valtteri Arstila. Peter Ross helped us to discover a number of mistakes in conversation. We are grateful to the audience in a workshop on colors with Fiona MacPherson in Fribourg, on November 2004, we remember in particular helpful remarks by Davor Bodrozic, Fabian Dorsch, Gianfranco Soldati and Gian-Andri Toendury. Finally we would like to thank the audience, in particular Kevin Mulligan, for their contribution to the discussion after the talk given by one of the authors [MNR] at the Fifth European Congress of Analytic Philosophy, Lisbon, August 2005. 2 See Crane and Piantanida (1983) and Billock, Gleason and Tsou (2001). 3 To say that orange is phenomenally composed of red and yellow is to say that the color at issue looks necessarily (as we will argue) reddish and yellowish. Talk of phenomenal composition can be reduced to talk about looking x-ish and y-ish (where “x” and “y” stand for basic colors). If we say that orange is phenomenally composed while a pure red is not we do not imply that orange looks in any sense more complex than pure red. Nor do we wish to imply that in the case of orange there are two pure components simultaneously present. Seeing a phenomenally composed color is not like hearing an interval. In the latter case two distinct elements are simultaneously present in the experience. But this does not seem to be the correct description of the former case (this insight is the result of discussions with Achill Schnetzer in preparation of Nida-Rümelin & Schnetzer, 2004). 4 For more on this issue compare our answer to the fourth objection in section 10. 5 See (Hardin, 1984) p.492-493. 6 Although there are these two notions of phenomenal composition to be distinguished the need to use two different terms to express the two notions disappears once we accept the following principle: For every phenomenal kind of color experience there is a specific apparent property P such that to have an experience of the phenomenal kind at issue is to see something as having property P. For instance, to have an experience falling under the phenomenal kind of experiences of bluish red is to see an object as being bluish red. Whoever accepts this principle can describe phenomenal color appearances by reference to the color the object appears to have (where colors are understood as apparent properties). In other places we defend this principle. But the principle at issue cannot be presupposed in the present context since we wish to discuss one of its consequences: the claim that the same color is necessarily seen in the same way by different people with respect to phenomenal composition. This is the modal claim we discuss in the present paper and therefore cannot presuppose in the terminology used in our discussion. (For discussions of the principle see the book in preparation edited by Vivian Mizrahi, M. Nida-Rümelin, Juan Suarez and Achill Schnetzer (in preparation) and see M. Nida-Rümelin (2006c). 7 Suppose that we describe the phenomenal fact about Mary’s particular experience by saying that Mary sees a color that is phenomenally composed of red and violet. Assuming that Mary’s experience is an experience of the color violet it follows that the claim at issue about Mary’s particular experience cannot be true without the general claim (V) being true. But (V) expresses a modal claim about all violet experiences. It follows, paradoxically, that no particular descriptive claim about the phenomenal structure at issue of a given violet experience could be true without the truth of the corresponding modal claim about all violet experiences. But then the modal claim expressed in (V) states the necessary truth of particular claims that already presuppose the truth of (V) itself. This result can only be avoided by the explicit distinction between the two notions of phenomenal composition. For more about this point see M. Nida-Rümelin, 2005a. 8 The status of green in this respect has actually been an issue among philosophers and among scientists as well. See Schnetzer (2005), Brentano (1979a and 1979b) and Schulte (1987). For recent empirical research about the nature of green see Miller (1997) and references therein. Note that the distinction between two notions of phenomenal composition is necessary to avoid

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confusion in an analysis of this well-known debate. There are two issues involved rather than one: the issue about particular experiences (Brentano’s judgment about his own experiences of green for instance) and the issue about the color green (is it composed or ‘simple’?). The former issue involves questions about the reliability of introspection; the latter issue involves, in addition, epistemological questions concerning modal claims about phenomenal structure. 9 For an elaboration and a critical discussion of this view see in Vivian Mizrahi, M. Nida-Rümelin, Achill Schnetzer & Juan Suarez, “Philosophy of color vision science”, chapter 2, in preparation. 10 Compare chapter 2 of op. cit. in footnote 6. 11 The necessity at issue is a de re necessity in this sense: it is a necessary property of a certain type of experiences that each of its members has a certain phenomenal structure. 12 We sometimes talk of (V) as being a modal claim and sometimes as only implying a modal claim. The latter formulation can be motivated by pointing out that while it is arguably unavoidable to assume that (V) cannot be true unless the corresponding claim about color experiences is true, the thesis that the content of the assertion (V) is exhausted by that modal claim about phenomenal structure would require a further argument. However, if (V) implies the modal claim about phenomenology then we may also say that it is a modal claim about phenomenology even if there might be more to its content. Seen in this way the two formulations are both admissible and are not in conflict. 13 Note that the fact that green and red when mixed on a piece of paper may appear grey or brownish is no objection. These cases are not cases of phenomenal compositions in the sense at issue. (Nida-Rümelin & Schnetzer, 2004). 14 (Crane & Piantanida, 1983) 15 (Billock et al., 2001). The perception of so-called ‘novel colors’ has also been reported by synaesthetes. Color-blind subjects reported seeing synaesthetic ‘Martian colors’, normal subjects have reported the perception of ‘new colors’. Fiona Macpherson (personal communication) has drawn our attention to these reports. Cf. (Ramachandran & Hubbard, 2001); (Ramachandran, 2001) 16 Cf.(Thompson, 1992) and (Macpherson, 2003). 17 Cf. (Hardin, 1993) p.22. 18 For technical information cf. (Billock et al., 2001) p.2402. For the specific sort of eyetracker used in the experiments cf.(Crane & Steele, 1985). 19 For interesting ‘do it yourself’ experiments involving the blindspot cf. http://faculty.washington.edu/chudler/chvision.html 20 The complete story about the luminance pathway is of course more complicated. Cf. (Cavanagh, 1991) for a review on empirical work on the luminance pathway. Cf. (Billock & Tsou, 2004) for a general account of the role of luminance in visual perception. 21 In (Crane & Piantanida, 1983) the only information available is that more than 12 subjects were submitted to the stimulus. 22 For the subject reports see (Billock et al., 2001) pp. 2398-2399. We have put between parentheses the number of subjects per percept. This information was available only for the study of Billock et al. 23 Cf. (Dacey & Packer, 2003) and (Lennie, 2000) for reviews on this subject. 24 As reported in (Billock et al., 2001) p.2398 and (Hardin, 1993) p.xxix. 25 In the second edition of Hardin’s Color for Philosophers p.xxix. Cf. also (McLaughlin, 2003) pp.113-114. 26 Apparently Crane and Piantanida knew also that the experiment worked better at equiluminance: “By the way, when I called Hewitt Crane to tell him about our results, he told me that he and Piantanida knew that the forbidden color effect worked better with equiluminant colors, but they never wrote a follow-up paper on the effect, so they had no venue to publish the fact in.” Vince Billock (Private communication). 27 Private Communication. 28 One of the two authors (MNR) claims having discovered that experiences of the kind just described occur (or can occur) when looking through goggles with a red and a green glass on a white surface. The present speculation is motivated by this personal experience. The skeptical reader is invited to conduct this simple experiment. The other author (JS) is quite skeptical about

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this claim and does not wish to give much weight to phenomenological observations of one’s own particular case in self-experimentation of this non-scientific kind. This author finds it hard to accept that the experts in color vision involved in the experiments were unable to distinguish the described phenomenon from genuine phenomenal composition. In addition, according to JS, the case of gradient color (see reports of kind 2) does not seem to be explicable by the error theory that has been proposed. In view of these considerations the error theory at issue has very low plausibility according to JS. 29 This claim is controversial. See (Sternheim & Boynton, 1966); (Boynton & Olson, 1990) for linguistic studies showing that green is a primary hue and (Gordon, Abramov, & Chan, 1994) for a defense of the methodology. For Brentano about this issue see (Nida-Rümelin & Schnetzer, 2004) (Schnetzer, 2005). For a non-realist position about phenomenal composition in general and about the issue of the uniqueness of green see (Mizrahi, 2005). 30 Pseudonormal people have a red experience when they see a green object and a green experience when they see a red object. This phenomenal difference is caused by a physiological difference between pseudonormally and normally sighted people. It is controversial whether there could be actual pseudonormal people given what we know about the way our visual system functions. Whether pseudonormal people represent a pertinent possibility for us (in this sense, whether they are possible) depends on the way our visual system functions. For pseudonormality cf. (Nida-Rümelin 1999). 31 I then misapply the term but do I also misapply the corresponding concept? The answer is a bit tricky: I associate the wrong concept with the term but I do not misapply the phenomenal concept. For the case of pseudonormality see (Nida-Rümelin, 1999), for the impossibility of misapplying phenomenal concepts see (Nida-Rümelin, 2006). 32 One of the authors (JS) agrees with the thesis that in phenomenal cases, the criterion we use in actual cases is the same as the one we use in counterfactual cases, but thinks we might be in a bad position to know what the criterion is. This author thinks that the phenomenal criterion we actually posses might be such that it cannot be adequately revealed on the basis of a limited range of experiences, more specifically, that it cannot be revealed without testing the problem cases, many of which might not be covered in the actual range of experiences. As a consequence, it is possible, though unlikely, that the commonality that phenomenological reflection reveals as present in all actual experiences of violet is not a necessary part of the criterion to classify an experience as violet, even though the presence of the commonality might make it seem very natural to think of the having of the commonality in question as a requisite to classifying an experience as violet. 33 These claims are closely related to the view about phenomenal concepts developed in (Nida-Rümelin 2003 and 2006b). 34 The same intuitive idea is formulated by Kripke in his famous claim that the concept of pain fixes its reference by way of a non-contingent reference fixer. According to one of the authors the idea is however better captured in a framework that differs from Kripke's, see (Nida-Rümelin, 2006b), last section and note 35 below. 35 In the preceding section we introduced a necessary and sufficient condition at the corresponding place where the condition was that the experience is phenomenally composed of red and blue where none of the two components prevails too much. We skip the last part about the predominance of one of the components here and talk of a necessary condition only since this is what we aim at in the argument: we wish to justify the modal claim that violet contains red and blue and this modal claim concerns the relevant necessary condition only. 36 In this step we presuppose the following principle: Necessarily a being has property P1 if it has property P2 iff in all counterfactual circumstances a being that has P2, also has P1. 37 The conceptual fact used here has been expressed in the two dimensional framework saying that these concepts have identical primary and secondary intension see (Chalmers, 1999) or that they are actuality-independent, see (Nida-Rümelin, 2006b). Note that this is true only for phenomenal concepts and arguably false for the corresponding phenomenal terms (this point is elaborated in my book “Thoughts about Experiences”, OUP, in preparation). This difference, however, does no harm to the above argument which is – for that reason – formulated with respect to concept extension (and not with respect to extension of terms). The argument would of

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course go through in a version that uses the extension of linguistic terms but it would have to be formulated in a slightly different and a bit more complicated manner. For more about the claim at issue concerning counterfactual extension of phenomenal concepts compare M. Nida-Rümelin, 2006a. 38 For a theory about rational intuition see e.g. (Bealer, 2000) 39 According to the realist claim mentioned at the beginning of the paper people can be wrong about the phenomenal structure of their own present color experiences. In our answer to the first objection we use a further fallibilist claim: we can be wrong about the phenomenal criterion used in our own applications of phenomenal concepts. 40 We owe this objection to Fiona Macpherson who formulated it (without endorsing the argument) in personal communication.

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