Post on 23-Feb-2023
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COLOUR PHYSICALISM, NAÏVE REALISM, AND THE ARGUMENT FROM STRUCTURE
Keith Allen, University of York
keith.allen@york.ac.uk
Forthcoming in a special issue of Minds and Machines edited by Mazviita Chirimuuta.
Please refer to the published version.
Abstract: Colours appear to instantiate a number of structural properties: for instance,
they stand in distinctive relations of similarity and difference, and admit of a fundamental
distinction into unique and binary. Accounting for these structural properties is often
taken to present a serious problem for physicalist theories of colour. This paper argues
that a prominent attempt by Byrne and Hilbert (e.g. 2003) to account for the structural
properties of the colours, consistent with the claim that colours are types of surface
spectral reflectance, is unsuccessful. Instead, it is suggested that a better account of the
structural properties of the colours is provided by a form of non-reductive physicalism
about colour: a naïve realist theory of colour, according to which colours are superficial
mind-independent properties.
1. The Argument from Structure
Colours appear to instantiate a number of characteristic ‘structural’, or ‘higher-order’,
properties. First, they stand in distinctive relations of similarity, difference, and
exclusion: for instance, orange is more similar to red than orange is to blue, no shade
of yellow is a shade of red, and so on. Second, the colours admit of a fundamental
distinction into unique and binary (or compound more generally). Unique colours are
‘pure’ or ‘perceptually unmixed’ instances of the hues yellow, red, green, and blue and
the achromatic colours black and white. Binary colours, by contrast, are always a
‘perceptual mixture’ of two other unique colours: for instance, orange is a perceptual
mixture of red and yellow, cyan is a perceptual mixture of blue and green, grey is a
perceptual mixture of black and white, and so on. (Compound colours more generally
are perceptual mixtures of more than two unique colours: for instance, brown is
perceptual mixture of red, yellow, and black.)
The structural properties of the colours are often taken to have a special status.
The structural properties are not merely contingent properties: properties that the
colours could have failed to instantiate. Rather they are necessary properties—
properties that the colours instantiate in all possible worlds in which they exist—or
perhaps even essential properties—properties which serve (at least in part) to
individuate the colours, and in virtue of which the colours are the things that they are,
and not anything else.1 On this view, orange (for instance) is necessarily more similar
1 The claim that the structural properties are necessary properties of the colours is not always sharply
distinguished from the claim that they are essential properties. But necessary and essential properties
are not obvious identical: as Fine (1994) argues, Socrates is plausibly necessarily, but not essentially, a
2
to red than blue and a perceptual mixture of red and yellow, and indeed part of what
it is for a colour to be orange is for it to be more similar to red than blue and a
perceptual mixture of red and yellow: orange simply would not be the property it is if
did not instantiate just these structural properties. The view that the structural
properties of the colours are essential properties of the colours is described as a core
common sense belief about the colours by Johnston, according to whom:
Thanks to its nature and the nature of the other determinate shades, canary yellow, like
the other shades, has its own unique place in the network of similarity, difference and
exclusion relations exhibited by the whole family of shades (1992: 138).
Similarly, defending the claim that the colours instantiate the structural properties
necessarily, Hardin argues that:
If we reflect upon what it is to be red, we readily see that it is possible for there to be a red
that is unique, i.e. neither yellowish nor bluish. It is equally apparent that it is impossible
for there to be a unique orange, one that is neither reddish nor yellowish. Since there are
necessary properties of hues, nothing can be a hue without having the appropriate
properties necessarily (1993: 66).
If the structural properties of the colours are necessary or essential properties of the
colours, this in turn sets constraints on what the colours could be: the colours will be
those properties that instantiate the relevant structural properties.2
The structural properties of the colours have been widely discussed in the
philosophical literature. It is misleading to talk of ‘the Argument from Structure’, since
the structural properties of the colours have featured in a number of related
philosophical arguments, each with slightly different premisses and conclusions.3
However, a common target of arguments that appeal to the structural properties of
the colours are views according to which colours are physical, min-independent, member of the singleton set {Socrates}. The essentialist claim seems to me to better capture the content
of the relevant intuition, although I will not argue for this here. Note that the weaker claim that the
structural properties are necessary properties of the colours is sufficient to set non-trivial constraints on
what the colours could be. 2 The claim that the structural properties of the colours are essential properties of the colours is
consistent with treating colours as kinds, assuming that kinds have certain properties essentially.
However, it is does not necessarily imply that colours are natural kinds, if this means that colours are
properties with a complex physical or micro-physical essence. According to the naïve realist theory of
colour defended below, colours are better thought of as superficial kinds. See also Author. 3 Some versions of the Argument from Structure do not rely on the claim that colours are necessary or
essential properties of the colours. The version of the argument presented here is more controversial in
this respect, but stronger as a result. The distinction between non-modal and modal versions of the
argument is noted by Cohen (2003); see §2 for some discussion.
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properties of things in the environment. Specifically, these arguments are typically
directed against reductive physicalist theories of colour, according to which colours are
(‘narrowly’) physical properties: for instance, types of surface spectral reflectance (e.g.
Byrne and Hilbert 1997, 2003, Tye 2000), or else microphysical properties of objects
(e.g. Jackson 1996). According to one way of developing this line of argument:
The Argument from Structure
1. The structural properties of the colours are essential properties of the colours; 2. There are no physical properties of things in the environment that instantiate
the structural properties of the colours; 3. If colours are physical properties of things in the environment, then everything
that is true essentially of the colours is true essentially of these physical properties;
4. Therefore, colours are not physical properties of things in the environment.
This general style of argument is employed by Berkeley’s spokesman Philonous in
response to Hylas’s physicalistic identification of sounds with motion:
PHILONOUS. …are you sure then that sound is really nothing but motion?
HYLAS. I am.
PHILONOUS. Whatever therefore agrees to real sound, may with truth be attributed to
motion.
HYLAS. It may.
PHILONOUS. It is then good sense to speak of motion, as of a thing that is loud, sweet,
acute, or grave. (Berkeley 1713/1734: 172).
More recently, this form of argument has been presented in response to reductive
physicalist theories of colour by Hardin (1993), Maund (1995), Thompson (1995),
Pautz (2006), and Dorsch (2010) amongst others.4
After briefly considering premisses (1) and (3) of the Argument from Structure
in §2, §3 considers in detail a prominent physicalist attempt to reject premiss (2). I will
argue that this response is ultimately unsuccessful, and in §4 present a different way of
rejecting premiss (2) that appeals instead to a form of non-reductive physicalism about
colour: a naïve realist (or primitivist) theory of colour according to which colours are
superficial mind-independent properties.
4 Related argument with epistemic premisses are presented by Boghossian and Velleman (1991) and
Johnston (1992). Johnston, for instance, argues that if colours are physical properties, then their
structural properties could only be discovered by empirical investigation. But this violates a what he
takes to be core common sense belief about the colours, Availability, that ‘Justified belief about the
canary yellowness of external things is available simply on the basis of visual perception’ (1992: 138).
The advantage of the present argument is that it does not appeal to this principle, which it would be
open to physicalists to reject.
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2. Premisses (1) and (3)
One response to the Argument from Structure is to reject premiss (1). A popular way
of denying premiss (1) consistent with a physicalist theory of colour is to deny that
colours instantiate the structural properties essentially, by distinguishing between
colours and colour experiences and reinterpreting premiss (1) as a claim about our
colour experiences. On this view, the colours themselves may still instantiate various
structural properties in virtue of structural properties of the experiences that these
properties produce; however these properties are essential properties only of colour
experiences, and not of the colours themselves. A consequence of this is that the
properties identified with the colours could produce different experiences with
different structural properties, and so could themselves stand in different relations of
similarity and difference (e.g. Lewis 1997; Cohen 2003; McLaughlin 2003).5
One advantage of this approach is that it provides a way of recognising the
seemingly close relationship between the way colours appear and the way that the
visual processing mechanisms underlying colour experience function (although I
return to this in §4). At the same time, this view goes at least some way towards
respecting the appearances, at least to the extent that colours are, as they appear to
be, mind-independent properties of things in our environment.6
The main problem with this view is that it appears to get things the wrong way
around. On the face of it, colour experiences appear to stand in distinctive relations of
similarity and difference because of relations of similarity and difference between the
colours, and not vice versa. This follows naturally from the phenomenological claim
that perceptual experience appears to be transparent—that when we reflect upon our
experiences, we are only aware of objects, properties, and relations in the environment,
and not any properties of the experience itself—and hence that our experiences
appear to ‘inherit’ their qualitative or phenomenal character from that which they are
5 An alternative way of developing this general line of response is to argue that the structural properties
are properties of experience that are falsely attributed to the colours (e.g. Churchland 2007). This is
problematic for broadly the same reasons as the view considered in the text. A more radical response is
to deny that the structural properties of the colours are essential properties of anything at all, as
opposed to being artefacts of our linguistic representations (e.g. Saunders and van Brakel 1997; Mizrahi
2009). I will not address this form of relativism here, but for some relevant discussion see Jraissati
(2014). 6 The phenomenological claim that colours appear to be mind-independent properties of things in the
environment—rather than, say, mind-dependent dispositional or relational properties—is sometimes
denied (e.g. Cohen 2009; Chirimuuta 2011). However, for defences of this claim, see e.g. Author;
Roberts, Andow, and Schmidtke (forthcoming).
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experiences of.7 There is no obvious phenomenological difference in this respect
between colours and their structural properties: just as colours appear to be mind-
independent properties of things in our environment, the structural properties
associated with the colours appear to be mind-independent properties of properties of
things in our environment, not properties of our experiences themselves. Given the
transparency of experience, the claim that the structural properties are primarily and
essentially properties of colour experiences, and that colours only instantiate the structural
properties derivatively and contingently, is liable to seem to mislocate the locus of the
relevant essential truths. These considerations may not provide absolutely conclusive
reasons for rejecting this type of approach: for instance, proponents might be
prepared to accept these seemingly counterintuitive consequences (Lewis 1997), or try
to explain them away (Cohen 2003). Nevertheless, they provide a motivation for
considering alternative responses to the Argument from Structure.8
Premiss (3) of the Argument from Structure is an application of Leibniz’s Law.
Without denying Leibniz’s Law outright, an alternative response to the Argument
from Structure is to attempt to restrict its application in the present context—for
instance, by denying the claim that the structural properties of the colours should be
evident from the physical description of their nature. On this view, the structural
properties of the colours would be evident only on the basis of visual perception, even
though the colours would themselves be physical properties. The challenge for this
approach is to make sense of the fact that the structural properties are essential
properties of the colours, without conceding that they have a nature that is, at least in
part, not (narrowly) physical. On the one hand, if the structural properties of the
colours are evident only on the basis of visual experience because (for instance) visual
experience provides a contingent mode of presentation for the physical properties that
are identified with the colours, then this undermines the claim that the structural
properties are essential properties of the colours. On the other hand, if the structural
properties of the colours are essential properties of the colours that are not evident
from the physical perspective, then, as Byrne (2003: fn. 5) suggests, this begins to
sound more like a dual aspect theory than a full-blooded form of reductive
physicalism, since physicalists typically insist that a thing’s essential properties are
those that are captured by the physical description of their nature.9
7 On the ‘inheritance’ metaphor and its relationship to the transparency intuition, see e.g. Campbell
(1993); Shoemaker (2003); Kalderon (2011). 8 For more detailed discussion of problems with versions of this approach, see e.g. Byrne (2003); Pautz
(2006); Cohen (2009); Davies (2014). 9 Davies (2014) defends a view of this kind, comparing this response to ‘type-B’ physicalist accounts of
the mental. Davies’s defence of this approach is programmatic, noting that colour physicalists can
adapt whatever turns out to be the best type-B physicalist account of the mental. Assuming that a viable
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3. Premiss (2): Reflectance Physicalism
Given a presumptive motivation for accepting premisses (1) and (3), an alternative
response to the Argument from Structure is to reject premiss (2), the claim that there
are no physical properties of things in the environment that instantiate the structural
properties characteristic of the colours. This section considers one way of developing
this line of response; an alternative is suggested in §4.
3.1 Byrne and Hilbert’s Reflectance Physicalism
According to a popular form of colour physicalism, colours are identical with types of
surface spectral reflectance (SSR).10 SSRs determine the proportion of the incident
light that an object reflects at each visible spectral wavelength; colours are identified
with types of SSR to accommodate metamers, objects that differ in SSR but which
nevertheless appear identical in colour (at least in certain conditions). SSRs types are
individuated in terms of the relative proportions of the incident light that objects
reflect across different broadband regions of the visible (to humans) part of the
electromagnetic spectrum. Such properties are not very interesting from a physical
point of view, but are nevertheless mind-independent (Hilbert 1987). And although
surface spectral reflectances are relatively high-level physical properties—in contrast,
for instance, to their micro-physical realisers—this is still a form of reductive physicalism
to the extent that colours are identified with properties described by the physical
sciences.
On the face of it, SSRs do not appear to instantiate the characteristic
structural properties of the colours: for instance, if we consider sample SSRs for red,
orange, and blue, there need be nothing about these SSRs that would lead us to
expect that the first is more similar to the second than it is to the third; similarly, if we
compare the SSRs for unique red and orange there need be nothing that would
obviously justify the description of one as unique and the other as binary (Byrne and
type-B physicalism about the mental is forthcoming, it would still need to be shown that this approach
can be adapted to allow for experience to provide knowledge of essential structural properties of the
colours without collapsing into a form of a dual aspect theory. Type-B physicalists often appeal to
phenomenal concepts to explain the different mode of access to physical properties that experience
provides. However, phenomenal concepts are often taken to provide only a ‘thin’, demonstrative,
characterization of their referents, in which case it is difficult to see how they could provide substantive
knowledge of the essential structural properties of the colours. Alternatively, if phenomenal concepts
provide more substantive characterisations of their referents that allow for knowledge of their structural
properties not provided by physical concepts (as e.g. Schroer 2010 suggests), then it begins to sound as
though colours are properties whose essential nature is not fully physical. 10 Or to account for coloured light sources, types of productance more generally (e.g. Byrne and Hilbert
2003). However, I will focus on SSR’s here.
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Hilbert 2003: 13).11 Nevertheless, Byrne and Hilbert (1997, 2003; see also Tye 2000)
have argued that it is possible to account for the structural properties of the colours
consistent with the view that colours are types of SSR. I will focus here on Byrne and
Hilbert’s most recent proposal, which comes in two parts: first, a thesis about the
representational content of colour experience; and second, a thesis about the
properties that these experiences represent.
According to the first thesis, the representational content of colour experience
has an underlying complexity (see also Sundström 2013). Focusing in the first instance
on experiences of hues, colour experiences do not simply represent objects as having
determinate colours, but rather represent objects as having determinate colours in
virtue of having ‘proportions of hue-magnitudes’. Hue magnitudes are sets of
properties combined with ratio scales: functions that map these properties onto the
real numbers. There are four hue magnitudes: R, Y, G, B. Objects are represented as
having values of up to two of these hue magnitudes in different proportions: for
instance, an object instantiating a binary hue like orange might be represented as
having values of R and Y that are each 50% of its total hue; an object instantiating a
unique colour like unique green will be represented as having a value of a single hue
magnitude (in this case G) that is 100% of its total hue. Although Byrne and Hilbert
focus on hues, this account can be generalised by introducing achromatic magnitudes
Bl and W. Achromatic objects (white, gray, black) will be represented as having values
of one or both these magnitudes, whereas objects instantiating ‘tonal’ colours (e.g.
olive, pink, brown) will be represented as having values of up to two chromatic and
two achromatic magnitudes.
This thesis about the representational content of colour experience is then
combined with a thesis about the properties that these experiences represent. The
light that objects reflect can be characterised in terms of the degree to which it
stimulates the three types of cones (L, M, S) in the eye. The responses of these cones in
turn determine the way in which the retinal signal is processed in three opponently
organised psychophysical channels. To use a simplified model, L-M determines the
red-green signal (L-M > 0 red, L-M < 0 green), (L+M)-S determines the yellow-blue
signal ((L+M)-S > 0 yellow, (L+M)-S < 0 blue), and L+M determines the achromatic
signal (L+M > 0 white, L+M < 0, black).12 Using ‘L*’, ‘M*’ and ‘S*’ to refer to the
11 The microphysical properties of objects with which some physicalists identify colours appear even less
plausible candidates for instantiating the relevant structural properties. 12 It was initially thought that opponent cells in the laternal geniculate nucleus (LGN) discovered in the
1960’s were the neural realisers of the psychophysical opponent channels hypothesised by Hering to
explain (amongst other things) the unique-binary structure of the hues. However, it now seems that
experiences of the unique hues cannot be explained solely by the activity of opponent cells in the LGN
firing in response to signals from the retinal receptors, and a third stage of processing has been
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degree to which light of a certain kind stimulates the different cones in the eye, an
object has a value of the hue-magnitude R if and only if (iff) it has an SSR of the SSR-
type which would reflect light with a greater L-intensity than M-intensity (L*-M* > 0),
and the greater the difference, the higher the value of R. Similarly, an object has a
value of the hue-magnitude G iff it has an SSR of the SSR-type which would reflect
light with a greater M-intensity than L-intensity (L*-M* < 0), and the greater the
difference, the higher the value of G; an object has a value of the hue-magnitude Y iff
it has an SSR of the SSR-type which would reflect light whose L+M-intensity is
greater than its S-intensity ((L*+M*)-S* > 0), and the greater the difference, the
higher the value of Y; and an object has a value of the hue-magnitude B iff it has an
SSR of the SSR-type which would reflect light whose L-intensity is greater than its
M+L-intensity ((L*+M*)-S* < 0), and the greater the difference, the higher the value
of B.
This in turn grounds the following account of the structural properties of the
colours. Similarities between the colours are explained in terms of similarities in hue
magnitudes. For instance, orange is perceived to be more similar to red than it is to blue
because orange objects are perceived to have (say) a 50% value of the magnitude R
and red objects (if they are unique) are perceived to have a 100% value of the
magnitude R, whereas blue objects (if they are unique) are perceived to have a 0%
value of the magnitude R. Correspondingly, the property identified with orange is
more similar to the property identified with red than the property identified with blue
because the orange and red objects both reflect light whose L-intensity is greater than
its M-intensity (i.e. both have values of the magnitude R), whereas the blue object (if it
is unique blue) reflects light whose L-intensity is equal to its M-intensity.
Finally, the distinction between unique and binary colours can be explained by
reference to the proportions of hue magnitudes that objects are represented as having.
Objects instantiating binary hues are always perceptually represented as having non-zero
proportions of two hue magnitudes: for instance, orange objects appears both reddish
and yellowish because orange objects are represented as having non-zero proportions
of R and Y. Objects instantiating unique hues, in contrast, are always perceptually
represented as having 100% value of a single hue magnitude: for instance, objects that
are unique red appear neither bluish nor yellowish because they are represented as
having a proportion of the magnitude R that is 100% its total hue. Corresponding to
this, objects instantiating binary hues are such that they have non-zero values of two
hue magnitudes, whereas objects that instantiate unique hues have a non-zero value of
hypothesised, the neural basis for which has yet to be identified (e.g. De Valois & De Valois 1993;
Abramov 1997; for discussion, see e.g. Jameson and D’Andrade 1997; Jraissati 2014; Degenaar and
Myin 2014).
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just one hue magnitude. So, for instance, an orange object reflects light whose L-
intensity is greater than its M-intensity (L*-M* > 0) and whose L+M-intensity is
greater than its S-intensity ((L*+M*)-S* > 0), whereas an object that is unique red
reflects light whose L-intensity is greater than its M-intensity (L*-M* > 0) and whose
L+M-intensity is equal to its S-intensity ((L*+M*)-S* > 0).
If successful, Byrne and Hilbert’s account provides a way of denying premiss
(2) of the Argument from Structure consistent with the thought that colour
experiences inherit their qualitative character from that which they are experiences of.
The following subsections consider the two parts of Byrne and Hilbert’s proposal in
turn.
3.2 Do Experiences Represent Hue Magnitudes?
An obvious concern with the first part of Byrne and Hilbert’s proposal is that it
ascribes too great a complexity to the representational content of experience. After all,
experience is often taken to present colours as perfectly simple properties; as Locke,
for instance, puts it, ideas of colour are simple ideas ‘which being each in it self
uncompounded, contains in it nothing but one uniform Appearance’ (1690: II.ii.1).13 This
part of Byrne and Hilbert’s proposal therefore requires justification.
Byrne and Hilbert motivate the claim that experience represents objects as
coloured by representing them as instantiating different proportions of hue
magnitudes as follows. In the case of binary hues, they argue that 1) orange objects
(for example) appear to be a ‘mixture’ of red and yellow in the sense that they appear
to be both reddish and yellowish, and 2) they appear to be both reddish and yellowish
because they are perceived as having different proportions of the two hue magnitudes,
R and Y (similarly for the other binary hues). 1) is intended to rule out some
unsatisfactory ways of understanding ‘perceptual mixture’: for instance, that orange is
produced by physically mixing red and orange; that orange things appear to be both
red and yellow (that is, appear to have a conjunctive property—cf. Byrne 2003: fn. 38;
Byrne and Hilbert 2003: fn. 43); and that orange things appear to be composed of
smaller red and orange pieces, in the way that we might see both peas and beans in a
stew (to use an example from Hering 1920; cf Byrne and Hilbert 2003: fn. 43). 2) is
intended to give a specific account of the first part of the proposal. Byrne and Hilbert
motivate 2)—their explanation of the appearance of reddishness and yellowishness in
terms of hue magnitudes—as follows:
13 Even if you reject the Lockean simplicity intuition, it is further question whether colour experience
exhibits precisely the kind of complexity that Byrne and Hilbert ascribe to it. See Sundström (2013) for
discussion of two different ways colours might appear to be complex, only one of which corresponds the
complexity attributed by Byrne and Hilbert.
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It is natural to say, and subjects do say, that one colored chip has “more blue” and “less
red” in it than another, that a certain yellow chip has “no red and no orange” in it, that
any orange chip has “some red and some yellow” in it, and so forth. If subjects are asked
to estimate the “relative amounts of hues” in a stimulus (for example 40 percent red, 60
percent yellow), not only do they seem to understand the instruction, but they give
similar answers (2003: 14).
The judgments about relative hue proportions that Byrne and Hilbert refer to
are used in the Natural Colour System (NCS) to generate a colour ordering system
(see e.g. Hård et al 1996; Sivik 1997). The assumption appears to be that the ease and
consistency with which subjects are able to make colour appearance judgments of this
kind is best explained by the hypothesis that these judgments reflect the way that
colours are standardly represented in experience, and that subjects are able to make
these judgments by, in effect, ‘reading off’ the content of their experiences. In what
follows, I want to raise two concerns with this suggestion. First, the ease and
consistency with which subjects are able to make colour appearance judgments should
not be over-estimated. Second, Byrne and Hilbert’s is not the only available account
of these judgments.
On the first point, there are some important limitations to the ease and
consistency with which subjects are able to make judgments about the relative
proportions of colours that a stimulus contains. Intra-subjectively, consistency varies
both between subjects and between tasks: Shamey et al (2010), for instance, found that
intra-subject variability increases by around 50% if subjects are asked to make direct
judgments about the relative hue proportions that individually presented samples
contain, compared with a task in which subjects can compare and order a set of
physical samples presented simultaneously. Inter-subjective variation is greater than
intra-subjective variation. Inter-subjectively, judgments about relative hue proportions
are only roughly consistent (according to Sivik (2007: 180) within around 3-5% at a
.95 confidence-level), and there are differences, in particular, in the identification of
instances of the unique hues (Shamey et al 2010; Shamey et al 2011).14 The samples
used in the atlas that illustrates the NCS are the result of statistical analysis of the
cluster of responses that the hue estimation procedure produces (Sivik 1997: 179-180;
Matthen 2005: 89), and as a result the NCS atlas is unlikely to match all the
judgments of any particular perceiver. Consistency also varies across colour space.
Byrne and Hilbert focus on judgments about the relative proportions of hues in a
stimulus. But very few of the colours we normally perceive are fully chromatic, and in
14 For instance, Shamey et al (2011) found that the majority of subjects agreed with the physical sample
that the NCS describes as unique red, they mostly agreed with NCS unique green, but a majority
disagreed with the samples described by the NCS as unique blue and yellow. In each case, the mean
samples picked as representing unique hues varied between subjects.
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full generality their proposal will need to include judgments involving blackness and
whiteness: both for achromatic colours (black, white, and grey) and ‘tonal’ colours that
involve up to two hues and up to two achromatic colours (for instance, olive, brown,
pink). Focussing on judgments about relative hue proportions is liable to make colour
appearance judgments seem easier and more consistent than they generally are:
Shamey et al (2011), for instance, found that subjects find it significantly more difficult
to reliably judge the composition of achromatic colours (up to half erroneously
including chromatic components) and ‘tonal’ colours that involve up to four
components.
Besides, achieving these levels of consistency requires training participants for
up to fifteen minutes, and relies on using a ‘forced choice’ paradigm in which subjects
are prompted to describe colour samples only in terms of differing proportions of
unique instances of the six elementary colours identified by Hering: red, blue, green,
yellow, black, and white. The forced choice paradigm, however, dramatically reduces
variation. If no restrictions are placed on the list of basic colours which can be used to
describe samples, then the number of basic colours used increases. For instance, many
subjects find it difficult to describe brown samples using any other term than ‘brown’;
even Hering (1920) thought that identifying brown as orange (i.e. yellow and red) and
black was by no means obvious. Other potentially problematic colours include
orange, purple, and pink (Fuld et al 1982). It has been shown that subjects can
typically describe browns, oranges, purples, and pinks in terms of Hering’s elementary
colours if prompted. But the fact that prompting is required raises a doubt about
whether coloured objects are always represented in experience as having differing
proportions of the magnitudes B, Y, R, G; indeed, it might seem to lend some
support to the claim that they can be represented as having proportions of further
magnitudes, such as Br (brownish), O (orangish), Pi (pinkish), Pu (purplish). Green
raises the converse problem, because a number of people are inclined to describe
green in terms of blue and yellow.
Indeed, it is worth noting that even after training, some subjects are liable to
make ‘errors’ (from the perspective of the NCS) in describing colour appearance.
Shamey et al (2010), for instance, found that only 18 out of 30 subjects reliably
described hues in ways permitted by the NCS; the remaining subjects described some
colours in terms of opposing unique hues (for instance, green in terms of blue and
yellow), sometimes used three unique hues to describe a particular sample, or in two
cases simply had ‘conceptual problems with the task’. Moreover, whether all these
judgments are in fact ‘errors’ is itself controversial. Although now widely accepted,
there is a history of theoretical resistance to Hering’s identification of four elementary
hues, instances of which are unique. At the start of the twentieth century Brentano, for
instance, argued in response to Hering that green is phenomenally composed of
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yellow and blue (see Author for further discussion). More recently, the physicist Jan
Koenderink has suggested on phenomenological grounds that there might actually be
six basic hues: the Hering primaries plus cyan and magenta (2010: 579-81).15
Whatever the explanation of the relative ease and consistency with which some
trained subjects are able to make judgments about the proportions of hues a stimulus
contains, it needs to be consistent with ignorance, error, and variation amongst
normal perceivers who possess a full range of colour concepts. At the very least, the
existence of ignorance, error, and variation calls into question the view that colour
appearance judgments are simply a matter of ‘reading off’ the contents of perceptual
experiences.
This concern is made more pressing by the fact that Byrne and Hilbert’s
explanation of colour appearance judgments is not the only available one. A
competing explanation is suggested by a description of the judgments used in the
Natural Colour System by one of its principal proponents, Lars Sivik:
As references for [estimation and identification of colors] we are able, according to
Hering, to use the built-in images that we all have of ideal yellow, red, blue, green, white,
and black. The phenomenological basis of Hering’s postulates, to which we all have
access, is that all colors can be described in terms of resemblances to these six elementary colors…The
phenomenological-empirical basis for the NCS is thus that all colors (here I refer to
percepts of surface colors) more or less resemble these elementary colors….The unit of
measurement chosen for the variable “visual resemblance to the elementary color A” was
percent (1997: 174-175).
The view that this suggests is reminiscent of a view expressed by Hume:
It is evident, that even different simple ideas may have a similarity or resemblance to
each other; nor is it necessary, that the point of circumstance of resemblance should be
distinct or separable from that in which they differ. Blue and green are different simple
ideas, but are more resembling than blue or scarlet; though their perfect simplicity excludes
all possibility of separation or distinction (1739-40: 1.1.7)
According to the alternative account that these remarks suggests, orange objects
(for example) are both reddish and yellowish because orange things are like, to a
certain degree, both unique red and unique yellow things. But the resemblance of
orange to red and yellow is not to be further explained in terms of anything else. It is
15 According to Koenderink ‘I fail to see RED and BLUE in a PURE PURPLE, just as I fail to see RED and
GREEN in a PURE YELLOW’ (2010: 579). He argues further that the six basic hue model captures
alterations of brightness amongst the hues (that yellow, cyan, and magenta are relatively bright, and
red, green, and blue relatively dark), and allows for the basic hues to equally spaced around the colour
circle.
13
not to be explained in terms of orange objects sharing properties with red and yellow
objects: as, for instance, on Byrne and Hilbert’s (1997) earlier suggestion that orange is
similar to red because both have the coarse-grained determinable property of being
reddish. Nor is it to be explained in terms of orange and red objects being represented
as having different proportions of the hue magnitude R. Instead, the similarity is
basic: it is simply the case that orange objects are like, to different degrees, unique
yellow and unique red objects. From this perspective, when Byrne asks ‘The peach
and raspberry, one might think, don’t seem to share a color property. So how come
they look similar?’ (2003: 656), the call for explanation is misplaced.
This view still needs to explain the phenomenon that Byrne and Hilbert use to
motivate the appeal to hue magnitudes: some trained subjects’ abilities to make
relatively reliable judgments about colour appearance. Sivik’s suggestion is that we
have ‘built-in images’ of pure instances of the Hering primaries that we are able to
mentally compare presented samples with, and on this basis make judgments about
the relative degrees to which a presented sample resembles those built-in images.16
This might seem suggestive of what Wittgenstein describes as a ‘false picture’ of
recognition:
as if recognizing always consisted in comparing two impressions with one another. It is as
if I carried a picture of an object with me and used it to perform an identification of an
object as the one presented by the picture (1953: §604).
An alternative, in keeping with the basic thought that colour similarities are basic, is
that we employ recognitional capacities to make judgments about a sample’s degree of
resemblance to pure instances of the elementary colours, without explicitly needing to
bring images of these colours to mind (see e.g. Evans 1982 on recognitional
capacities,). Either way, judgments about colour appearance would go beyond what is
strictly speaking presented in experience, or at best constitute a form of ‘seeing as’ (see
e.g. Wittgenstein 1953, and Brewer 2011: §5.3 for one way of developing this idea).
As well as explaining how subjects are able to make colour appearance
judgments, this account also has the scope to explain how ignorance, error, and
variation are possible. Rather than simply ‘reading off’ information about colour
appearance from the content of experience, judgments about similarity relations are
only possible if these similarity relations are salient, and making these similarity
relations salient might require prompting—just as one person’s similarity to another
might only be noticed if you are prompted to direct your attention in the appropriate
16 Compare Shamey et al (2011) who conclude that ‘The above finding supports that uHs [unique hues] are a well established concept in the minds of subjects that have not had extensive exposure to the idea’. Describing these ‘cognitive references’ as ‘built-in’ suggests that they are innate; a slight variation on this would be to hold that they are learnt through experience.
14
ways, or images of the two people are placed side by side. Likewise, making judgments
about similarity relations involves making judgments about the resemblance of
samples to certain fixed points in colour space, but which points in colour space are
taken to be the fixed points is not simply determined by the content of the experience,
and might be something about which disagreement is possible—just as some people
are able to see resemblances between people that others cannot.17
The significance of this alternative account is that it provides some support for
the claim that colour similarity relations are, contrary to what Byrne and Hilbert
suggest, genuine (or natural) relations of similarity. Genuine similarity relations contrast
with relative similarity relations, which hold between things that are similar only
relative to a specific property or properties: for instance, in the way that squares and
triangles are similar relative to the properties of being geometrical shapes, having more than
three sides, being examples of relative similarity used by Byrne and Hilbert (1997: 274). Genuine
similarity relations are not relative in this way, and hold between things in virtue of
their intrinsic or essential natures: for instance, in the way that you might think that a
cube is more similar to a pentagon than a circle (cf. Byrne and Hilbert 1997: 274-275).
Byrne and Hilbert are keen to avoid the suggestion that the similarity relations
between the colours are genuine similarity relations, because the SSR types with
which they identify colours do not stand in similarity relations that are genuinely
similar, but at best stand in relations of relative similarity (Byrne 2003: 648; Byrne and
Hilbert 2003: 13). However, if the similarities between the colours are represented as
basic, then this provides a reason for thinking that colour similarity relations are
genuine similarity relations.
Of course, it might be suggested that it does not follow from the fact that
colour similarity relations are represented as being genuine similarity relations that they
are genuine similarity relations: colours might be relatively similar even if they are not
represented as such. There would at least be a tension for physicalists who deny
premiss (2) of the Argument from Structure in pursing this line of response. The initial
motivation for denying premiss (2) of the Argument from Structure (rather than
premiss (1)) was to be faithful to the appearances. But if we should prefer an account
of the structural properties of the colours according to which they are as they appear,
then we should prefer an account according to which they are genuine (rather than
relative) similarity relations if this is how they are represented. Nevertheless, Byrne
and Hilbert’s account is primarily an account about the nature of the colours, and it
might be suggested that their account of the way colours are represented is ultimately
dispensable. The more important question at this point is therefore whether Byrne 17 Compare Mizrahi (2009). Mizrahi defends a form of conventionalism about the unique/binary
distinction. However, the view that perceiving similarities between the colours involves a form of
‘seeing as’ does not entail conventionalism, if certain ways of seeing colours are privileged.
15
and Hilbert’s account of the structural properties of the colours is empirically
adequate.
3.3 Is Byrne and Hilbert’s Account Empirically Adequate?
Turning from the way colours are represented to the properties that the colours are
identified with, a key question is whether there are perceptual similarities between the
colours that are not preserved at the level of reflectance-types. One problem in this
respect are similarity relations between the unique hues themselves.
NCS colour space represents the four unique hues at the cardinal compass
points of the hue circle, to emphasise their psychological salience. However, the NCS
strictly speaking consists of four independent scales—red-yellow, yellow-green, green-
blue, blue-red—rather than one single dimension of hue.18 As such, it is not possible to
compare the end points of these scales themselves. This feature is reflected in Byrne
and Hilbert’s account. Because the unique hues are all represented as having 100% of
one of the four magnitudes (and therefore have no hue magnitudes in common), no
comparative judgments of similarity between the unique hues are themselves possible;
correlatively, there are no similarity relations between the reflectance-types with
which Byrne and Hilbert identify the unique colours.
This is problematic given that the unique hues themselves appear to stand in
asymmetric similarity relations (cf. Hardin 1993: 127-134).19 A number of
representations of colour space locate the unique hues at irregular points around the
hue circle. In Munsell colour space, for instance, the hue circle is divided into 100
perceptually equals steps (judged at the level of just noticeable differences), and
partitioned into five basic hue groups: the four elementary hues identified by Hering—
red, yellow, green, and blue—plus purple. The reason for this is that the number of
perceptually equal hue steps between the between the unique instances of Hering’s
elementary hues in the Munsell system varies considerably: from red to yellow it is 23,
from yellow to green it is 18, from green to blue it is 28, and from blue to red it is 31.
Purple is identified as an elementary Munsell hue so that the elementary hues can be
equally spaced around the perimeter of the hue circle.
The samples in the Munsell colour atlas are spaced according to colorimetric
rules of additive colour mixture using photometers and Maxwell disks, rather than
direct visual observation. As a result, multi-dimensional scaling (MDS) studies on the
Munsell system of colour classification have since revealed slight anomalies: for
18 As Sivik remarks describing the decision to arrange the hues ordered by the NCS around a circle,
rather than a square: ‘since the hue circle is such a well established concept, everybody yielded to this
convention. One should, however, be aware of the fact that the hue circle is conceptually not one scale
but four’ (Sivik 1997: 176) 19 This is noted as a potential problem by Byrne (1993: 661, n. 40) but no attempt is made to address it.
16
instance, colours between Munsell-Blue (‘5B’) and Munsell-Purple-Blue (‘5PB’) look
comparatively too similar to each other. Still, these MDS studies vindicate Munsell’s
basic partition of the hue circle into five, a feature preserved in the isotropic Optical
Society of America (OSA) colour space, for instance, which unlike the Munsell system
does give a strict preference to colour appearances over psychophysical data (Indow
1988: 463; Jameson and d’Andrade 1997: 299-301).
Consistent with this, Kuehni (2005) found that the distances between the mean
values for Munsell chips identified as unique varies across the unique hues. The
distance between the mean hues identified as unique green and unique blue was
approximately 8.5 Munsell hue steps. It was about the same between unique yellow
and unique red—although Kuehni reports having the impression that the mean green
and blue chips were more similar to each other than the mean yellow and red chips.
However, it was approximately 10 hue steps between the mean values for unique
yellow and unique green, and approximately 13 hue steps between unique blue and
unique red. (Compare Hume’s claim above that ‘Blue and green are different simple
ideas, but are more resembling than blue or scarlet’.)
So, not only is it questionable whether objects are represented as coloured in
virtue of being represented as having proportions of hue-magnitudes, but more
importantly Byrne and Hilbert fail to account for all the relevant relations of similarity
between the colours. As such, there are serious problems with the response to the
Argument from Structure.
4. Premiss (2): Naïve Realism
The previous sections motivated the claim that the structural properties are essential
properties of the colours, and argued that so understood a prominent form of
reductive physicalism is unable to adequately account for them. This section outlines
an alternative response to the Argument from Structure that, like the form of
reflectance physicalism considered in the previous section, denies premiss (2).
The Argument from Structure is directed in the first instance at reductive forms
of colour physicalism, according to which colours are ‘narrowly’ physical properties:
properties described by the physical sciences, such as surface spectral reflectances or
their micro-physical realisers. This argument is in turn often used to motivate one of a
range of broadly subjectivist theories of colour: forms of eliminativism according to
which things in the environment are not coloured at all (e.g. Hardin 1993; Maund
1995; Pautz 2006), or forms of relationalism according to which colours are relational
properties constituted in terms of perceiving subjects (e.g. Thompson 1995). However,
without further argument it doesn’t follow from the fact that there are no narrowly
physical properties of things in the environment that instantiate the structural
properties standardly attributed to the colours, that there are there are no physical
17
properties in a more inclusive sense of the word ‘physical’ that instantiate the
structural properties characteristic of the colours.
Naïve realist theories of colour (sometimes also called primitivist theories of
colour) agree with reductive physicalist theories of colour that colours are mind-
independent properties of things in the environment: properties of things whose nature
and existence is constitutively independent of conscious subjects.20 They differ from
reductive physicalist theories in maintaining that colours are superficial (or sui generis)
properties: they are properties that are not described by the physical sciences. But to
say that colours are superficial properties is not to say that there is no sense in which
colours are ‘physical’. First, colours as the naïve realist conceives of them are ‘physical’
in an undemanding, non-technical, sense of ‘physical’ according to which tables,
chairs, galaxies, people and the like are all ‘physical things’—in contrast, for instance,
to sensations, ghosts, or souls (cf. Stroud 2000: Chapter 3). Second, colours as the
naïve realist conceives of them are typically also ‘physical’ in the more technical sense
that they are normally taken to supervene on narrowly physical properties like surface
reflectance profiles. To say that the colours supervene on narrowly physical properties
is to say that there can be no difference in an object’s colour without a difference in its
narrowly physical properties. Supervenience can come in different strengths
depending, for example, on whether there can be no difference in an object’s colour
without a difference in its narrowly physical properties as a matter of nomological
necessity, or whether there can be no difference in an object’s colour without a
difference in its narrowly physical properties as a matter of metaphysical necessity.
Either way, supervenience promises a way of integrating higher-level properties like
colours into a broadly naturalistic framework, without simply identifying these higher-
level properties with narrowly physical properties: the higher-level properties depend
on, without being identical to, the underlying physical properties of objects. To say
that colours are superficial properties that are distinct from, but supervene on,
narrowly physical properties is therefore not to express a commitment to a form of
anti-physicalism, but just to reject forms of reductive physicalism. In this respect, naïve
realist theories of colour are equivalent to non-reductive physicalist theories of mind.21
Naïve realist theories provide a different way of responding to the Argument
from Structure; alternatively, the Argument from Structure can be understood as a
20 Primitivism is sometimes characterised as a view that comes in realist and eliminativist forms,
depending on whether colours are primitive properties that physical objects (actually) instantiate (e.g.
Pautz 2006). As I understand it, naïve realism is not neutral in this respect. For further discussion, see
e.g. Author. 21 Naïve realist theories of colour have become increasingly popular in recent years. Proponents include
Campbell (1993), Yablo (1995), Watkins (2010), and Dorsch (2010). For discussion, see Byrne and
Hilbert (2007).
18
way of motivating a naïve realist theory of colour.22 According to the naïve realist,
colours are superficial properties of things in our environment, and they instantiate
just those structural properties that they are perceived to. These structural properties
are, as they are often taken to be, essential properties of the colours, and the colours
themselves are those properties from which our colour experiences inherit their
phenomenal character. As such, the naïve realist theory provides a straight-forward
account of the structural properties of the colours.
Whether this is a viable account of the structural properties of the colours will
depend on the viability of the naïve realist theory of colour more generally. It is often
assumed that modern science has ‘shown’ that naïve realist theories of colour are
mistaken. I therefore want to conclude by briefly considering two ways that this
general line of argument against the naïve realist can be developed and resisted.23
One line of objection to the naïve realist theory of colour is an instance of the
more general ‘causal exclusion problem’ familiar from discussions of mental
causation. According to this line of argument, if colours were superficial mind-
independent properties, then they would be excluded from playing any causal role in
the production of colour experiences by an object’s narrowly physical properties, and
so there would be no reason to believe in their existence (e.g. Hardin 1993: 61;
Johnston 1992: 139; Jackson 1996). Detailed consideration of this argument lies
beyond the scope of this paper, but there are nevertheless a number of general reasons
for thinking that it is unconvincing. First, the argument threatens to over-generalise.
Worries about causal exclusion are not unique to naïve realist theories of colour, but
also arise for the dispositionalist and the reflectance physicalist: dispositional
properties threaten to be causally excluded by their physical grounds, and types of
reflectance profile not only threaten to be causally excluded by their tokens, but are
anyway relatively high-level physical properties that threaten to be causally excluded
by microphysical properties of objects (e.g. Byrne and Hilbert 2007: 84). The only
view left standing by this line of argument appears to be the view that colours are
22 Compare Dorsch (2010). Pautz (2006) uses the Argument from Structure to motivate a form of
eliminativist primitivism. However, Pautz’s eliminativism does not sit easily with the ‘Relational View’
of colour experience that he uses to motivate primitivism, according to which ‘the structural properties
of colours are simply inherited from (or amount to) the structural properties of the colours that we are
sensorily related to in having those colour experiences’ (2006: 543; see also §2 above). The most natural
interpretation of the Relational View is that the properties of objects that we are sensorily related to in
perception are actually instantiated by those objects. In denying this, Pautz appears to accept an error
theory of the general kind that he is otherwise keen to avoid. 23 A different line of objection focuses on the claim that colours are mind-independent, which
represents common ground between the naïve realist and the reductive physicalist. I consider attempts
to undermine this claim by appealing to variations in perceptual experience elsewhere (Author).
19
micro-physical properties of objects, and the sheer strength of this conclusion might
seem to be suggestive of a problem with the argument rather than a reason to
embrace its conclusion. Second, and more positively, insofar as naïve realism is a form
of non-reductive physicalism, naïve realists can co-opt any one of a variety of
responses by non-reductive physicalists to parallel concerns about the causal exclusion
of mental states by their physical realisers (e.g. Campbell 1993; Yablo 1995; Watkins
2010).
A second line of objection is more closely connected to the issues discussed in
this paper, and concerns the seemingly close connection between the way colours
appear and the nature of our perceptual processing mechanisms. There might be
seem to be a deep tension, if not an outright incompatibility, between acknowledging
the close connection between the way our perceptual processing mechanisms function
and the way colours appear on the one hand, and the view that the structural
properties of the colours are essential properties of mind-independent entities on the
other. As Hardin, for instance, trenchantly puts the point:
so much about the colors—particularly the division between unique and binary hues—is
manifestly bound up with the peculiar characteristics of the human visual system that it
seems a vain pursuit indeed to search for a parallel set of structures in the general order
of nature. One might as well set about earnestly constructing improved Ptolemaic
epicycles (1993: 67).
In this respect, it might seem that anything other than a broadly subjectivist theory of
colour—or at any rate, one that acknowledges the subject’s contribution to the
phenomenal character of experience (like the view discussed in §2)—is inconsistent
with what scientific investigation has revealed about the how our visual processing
mechanisms function.
The fundamental problem with this line of argument, however, is that it
doesn’t follow from the fact that the way colours are perceived depends on the way
our visual processing mechanisms function, that the nature and occurrence of colour
experiences can thereby be explained solely in terms of those mechanisms. After all,
the way that we perceive anything depends upon the way that our visual processing
mechanisms function, and it does not obviously follow from this that all perceptual
experiences can be explained solely in terms of those mechanisms, without any
reference to the objects, properties, and relations in the perceiver’s environment that
those experiences purport to be experiences of. An alternative way of thinking of the
visual processing mechanisms is to think of them as enabling conditions that make
possible the perception of colours. So understood, explanations of the phenomenal
character of colour experiences in terms of the way visual processing mechanisms
function do not exclude explanations of the phenomenal character of colour
20
experiences that appeal to the properties of objects; rather they presuppose them. As
Kalderon, for instance, puts it:
Let us grant that the spectral power distribution of the light reaching the perceiver’s eye
and its retinal effects, as well as the subsequent, cascading effects of further visual
processing, at least partly determine the phenomenal character of the perceiver’s colour
experience. However, they do so, in part, by determining the fact that the appropriately
situated perceiver stands in the relevant relation to the qualitative aspect of the colour
presented (2011: 257).
5. Conclusion
Accounting for the structural properties of the colours provides a widely discussed
problem for physicalist theories of colour. This paper has raised some objections to a
prominent reductive physicalist attempt to account for the structural properties, and
outlined an alternative form of non-reductive physicalism: a naïve realist theory of
colour according to which colours are superficial mind-independent properties. I have
argued that this alternative approach allows us to take the chromatic appearances at
face value, without ignoring or denying the scientific facts about the causal processes
underlying colour perception.24
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