Physics Does Not Entail Causal Closure

Consider the class of all physical truths; that is, all

truths expressible using terms taken from an idealized, completed

science of physics (together with logical and mathematical

terms). The class includes all truths describing particular

matters of physical fact and also all truths describing physical

laws. In what follows I argue that this class of all physical

truths—physics, broadly construed—does not entail that the

physical realm is causally closed. After clearing the ground for

my argument in sections 1 and 2, I present the argument itself in

section 3. If my argument is sound, it presents a problem for

those philosophers like David Lewis and Daniel Dennett who

advance a causal argument for physicalism. This implication is explored

in sections 4 and 5.

1. Preliminaries

We begin with preliminaries. First, we will be working with

the following formulation of the causal closure thesis, taken

from Jaegwon Kim.

[Closure]: If a physical event has a sufficient cause at time t, it has a sufficient physical cause at t.1

1 Kim (2005: 15), except that ‘sufficient’ is added following Loewer (2007: 262, n. 28). To simplify the discussion we will assume determinism, and so whenever an event has a cause it has a sufficient cause.

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Second, on the relevant sense of entailment, a class of

propositions, C, entails a proposition, p, just in case there is

no logically possible world in which all the members of C are

true while p is false. Third, it bears emphasis that my thesis is

not that [Closure] is false. In fact, I am a physicalist who

accepts [Closure]. My thesis is just that the truth of [Closure]

does not follow from physics alone. Fourth, there is a question

of whether [Closure] itself qualifies as a physical truth, given

how we are understanding the notion of physical truth in the

paper (i.e., any truth expressible using just terms taken from

physics, logic, and math). If it does qualify, then [Closure] is

of course trivially entailed by the class of all physical truths,

in which case my thesis as stated is trivially false. I have two

points of reply to this concern.

First, my view is that [Closure] does not qualify as a

physical truth on the grounds that the term ‘cause’ (and causal

expressions more generally) will not figure in an idealized,

completed physics. This is a familiar thought originally and

influentially advanced by Bertrand Russell.2 Second, even if this2 Russell (1913); for recent discussion, see the essays in Price and Corry (2007). As Field (2003) observes, one need not be an eliminativistabout causation (as Russell was, at least at that point in his career) to hold that causation will not figure in a completed physics. You mightinstead hold that causation is real but ultimately reducible to

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Russellian suggestion is rejected, I have a fallback position. If

[Closure] qualifies as a physical truth, just reinterpret my

thesis as the claim that [Closure] is not entailed by the class

of other physical truths; that is, the class of all physical

truths minus those physical truths that are explicitly causal in

the way [Closure] is. This class of remaining truths will include

truths about the transference of energy-momentum, the structure

of light cones, and so on. If it can be shown that these truths

fail to entail [Closure], that will be enough for my purposes.

2. Negative Existentials and Double Prevention

My argument that physics does not entail causal closure

relies on two key premises. The first is that negative existential truths

are not generally entailed by the class of physical truths. Here

is a paradigmatic example of a negative existential.

[No Ectoplasm]: There is no nonphysical stuff, ectoplasm.

In connection with this first premise, I claim there is a

logically possible world that is physically indiscernible from

the actual world but differing in that it contains ectoplasm

while the actual world contains none. All the physical truths of

the actual world obtain at this possible world, and yet [No

Ectoplasm] is false there. Given that such a world is possible,

something physical and non-causal.

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it follows that even though [No Ectoplasm] is true at the actual

world, its truth is not entailed by the class of all actual

physical truths.

This first premise is fairly uncontroversial. Consider:

physicalism is often formulated as the thesis that all truths are

entailed by the physical truths taken together with a that’s-all truth

stating something to the effect that the actual world contains

nothing more than it must contain for all the physical truths to

obtain.3 The that’s-all truth is added in recognition of the

point that certain negative existentials like [No Ectoplasm] are

not entailed by the physical truths alone. Anyone who accepts

this familiar point from discussions of physicalism accepts my

first premise.

The second key premise of my argument is that negative events—

which I do not distinguish from absences or omissions—can enter

into causal relations. Of particular interest to us is the

phenomenon of double prevention.4 In the stock example, Bomber is on

a mission to destroy Target. Enemy hopes to prevent Bomber from

doing so, and would succeed in preventing this if left alone. But

3 See for instance Jackson (1998), Chalmers (1996) and (2012). Here, I bracket indexical truths (for instance, truths using terms like ‘I’ or ‘now’), which arguably also need to be included in the entailment base.4 For discussion, see for instance Hall (2000) and (2004), Lewis (2000) and (2004).

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Fighter, the escort of Bomber, prevents Enemy from preventing

Bomber from destroying Target—a double prevention—by shooting

down Enemy before Enemy can interfere with Bomber. With Enemy out

of the way, Bomber goes on to successfully destroy Target.

In connection with this second premise, I claim that the

event that is Fighter’s shooting down Enemy is a non-redundant

cause of the effect that is the destruction of Target. After all,

there is counterfactual dependence between the two events: if

Fighter had not shot down Enemy, Enemy would have shot down

Bomber, in which case Target would not have been destroyed. And

yet, Fighter causes the destruction of Target without shooting

Target herself, without transferring any energy or momentum to

Target or otherwise being physically connected to it, and despite

it being the case that the actions Bomber would have taken if

neither Enemy nor Fighter had been present are physically and in

all other respects indiscernible from the actions Bomber in fact

took in their presence.

This second premise will be accepted by typical proponents

of counterfactual theories of causation,5 but not by defenders of

certain opposing theories, including those that require causes to

5 For instance, Lewis (1973) and (2000).

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be connected to their effects via some physical process.6 My plan

is simply to assume without serious argument that such physical

process theories of causation are mistaken. Or, put another way,

I will be content if I can establish that the physical truths do

not entail [Closure] unless substantive and controversial

assumptions about causation are made, including the rejection of

counterfactual theories that recognize double prevention as a

form of causation.

3. Ectoplasm and Prevectoplasm

With these premises in place, consider some purely physical

causal interaction in the actual world in which one physical

event causes another. For instance, suppose that in a game of

billiards a cue ball collides with an 8-ball, causing the latter

to fall into the corner pocket. We can suppose that this purely

physical interaction is completely described by the physical

truths of the actual world.

Next, consider a world w that resembles the actual world in

various ways but differs in that it contains nonphysical

ectoplasm. The ectoplasm is causally potent, often interacting

with physical events. At w, a counterpart billiard game takes

place. When the cue ball at w collides with the 8-ball, the

6 See for instance Salmon (1984), Dowe (2000).

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ectoplasm intervenes, stopping the 8-ball in its tracks and

preventing it from reaching the corner pocket. Thus, w differs

from the actual world with respect to particular physical matters

of fact: the 8-ball reaches the corner pocket in the actual world

but not at w. But also, we can suppose, w differs from the actual

world with respect to physical law: when the ectoplasm

decelerates the 8-ball at w, it reduces the energy of the entire

physical realm there, and it also reduces the linear momentum of

the entire physical realm. We can suppose that this is in

violation of the physical conservation laws that obtain in the

actual world, the conservation of energy and the conservation of

linear momentum. I take this sort of violation of these

conservation principles to be a logical possibility—w as we have

described it is a logically possible world—even if the principles

are among our best-confirmed laws here in the actual world.

Finally, consider a third world, w*, that is physically

indiscernible from the actual world but contains two distinct

nonphysical substances, ectoplasm and prevectoplasm. When the cue

ball at w* collides with the 8-ball there, the ectoplasm is just

about to intervene and prevent the 8-ball from reaching the

corner pocket, exactly as happens in w. But the prevectoplasm at

w* prevents this from happening—a double prevention. Acting as a

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kind of escort for the 8-ball, the prevectoplasm “shoots down”

the ectoplasm, as it were, before the ectoplasm can interfere

with the 8-ball.

If it helps to attribute minds to these nonphysical

substances, you can think of prevectoplasm as a nonphysical mind

that wants the 8-ball to reach the corner pocket, while ectoplasm

is a nonphysical mind that wants this not to happen.

Alternatively, we could tweak the example by thinking of

ectoplasm and prevectoplasm not as distinct substances, but as

distinct nonphysical mental modes (property instances) within a

single nonphysical substance. The prevectoplasmic mode is a

desire this substance has for the 8-ball to reach the corner

pocket, while the ectoplasmic mode is a competing desire for this

not to happen. The nonphysical mind in question is conflicted

about what it wants. Ultimately, its desire for the 8-ball to

reach the corner pocket wins out, and the nonphysical mind allows

this to happen.

Again, the argument supposes that w* is physically

indiscernible from the actual world. It is indiscernible with

respect to all particular matters of physical fact. For instance,

at w* the 8-ball falls into the corner pocket, just as it does in

the actual world. It is also indiscernible with respect to all

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physical laws. For instance, the principles of the conservation

of energy and the conservation of linear momentum are not

violated at w*, as they are at w, given that the 8-ball at w* is

not decelerated. If we suppose that the interaction between

prevectoplasm and ectoplasm was itself governed by a

(nonphysical) law, we can suppose that violations of these

conservation principles are nomologically impossible at w*. That

is, we can suppose the conservation of energy and the

conservation of linear momentum hold as matters of law at w*,

just as they do in the actual world.

And yet, I claim, despite this complete physical

indiscernibility from the actual world, the prevectoplasm is a

non-redundant cause of the 8-ball falling into the corner pocket

at w*. For, if the prevectoplasm had not acted, the 8-ball would

not have reached the corner pocket—the ectoplasm would have

prevented it from reaching.

Superficially, the scenario we are imagining might seem to

resemble certain cases of causal overdetermination. In the stock

example of overdetermination, a firing squad executes a prisoner,

with each of the many bullets fired entering the prisoner’s body

simultaneously. Each bullet is by itself a sufficient cause of

the prisoner’s death, and so the death is causally

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overdetermined. For any particular bullet, it will be the case

that if that bullet had not been fired, the prisoner’s death

still would have occurred, given the other bullets. Similarly, it

might seem, at w* it will be the case that if, counterfactually,

nothing nonphysical had existed, the 8-ball still would have

reached the corner pocket, just as it does in the actual world.

From this counterfactual, you might mistakenly infer that any

nonphysical causes of the 8-ball reaching the corner pocket must

be nothing more than redundant, overdetermining causes.

The reason this inference would be mistaken is that when at

w* we counterfactually subtract away everything nonphysical, we

subtract away not only a nonphysical cause of the 8-ball’s

reaching the corner pocket (the prevectoplasm), but also a

nonphysical would-be preventer of this effect (the ectoplasm),

and the nonphysical cause in question makes its non-redundant

causal contribution to the effect precisely by preventing this

would-be preventer from preventing the effect. In standard cases

of overdetermination, there is a failure of counterfactual

dependence: if a given bullet had not been fired, the prisoner

still would have died. In our scenario, however, there is such

counterfactual dependence: given the presence of ectoplasm, if the

prevectoplasm had not acted, the 8-ball would not have reached

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the corner pocket. Thus, the prevectoplasm’s action is a non-

redundant, non-overdetermining cause of this physical effect, a

cause without which the effect would not have occurred.

I dwell on this point because some mind-body dualists have

proposed that all mental causation involves causal

overdetermination. Whenever a nonphysical mental event causes

some physical effect, these dualists hold, that physical effect

also has an independent, overdetermining, sufficient physical

cause. 7 These dualists accept [Closure]. Indeed, their aim is to

reconcile [Closure]’s truth with interactionist dualism.

But this is different from our scenario, since [Closure] is

false at w*. The physical effect that is the 8-ball’s falling

into the corner pocket there is non-redundantly caused by a

nonphysical event, namely, the act performed by prevectoplasm,

and so this physical effect does not have a sufficient physical

cause at the given time. But every physical truth that holds in

the actual world also holds at w*, where [Closure] is false.

Therefore, the class of all actual physical truths does not

entail [Closure]. Physics, even when it is construed as broadly

as possible, so as to include all actual physical truths, fails

to entail that the physical realm is causally closed.

7 See for instance Mills (1996) and Árnadóttir and Crane (2013).

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The physical truths of the actual world do not by themselves

entail [Closure], but I assume that [Closure] is entailed by the

physical truths taken together with the sort of that’s-all truth

mentioned above, stating something along the lines that the

actual world contains nothing more than what it needs to contain

for these physical truths to obtain. A that’s-all truth of this

sort would rule out the existence of ectoplasm and prevectoplasm.

However, this result is not the interesting, philosophically

controversial thesis that physics entails [Closure]. It is the

uninteresting, uncontroversial thesis that physicalism entails

[Closure], something that no dualist denies.

4. Dennett, Lewis and the Connection Between Physics and

[Closure]

If the preceding argument is sound, much that philosophers

have written about the connection between physics and [Closure]

is mistaken. Here I will focus just on two especially prominent

examples. In Consciousness Explained, while making his case against

interactionist mind-body dualism, Daniel Dennett writes,

No physical energy or mass is associated with [nonphysical minds]. How, then, do they get to make a difference to what happens in the brain cells they must effect, if the mind is to have any influence over the body? A fundamental principleof physics is that any change in the trajectory of any physical entity is an acceleration requiring the expenditureof energy, and where is this energy to come from? It is this

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principle of the conservation of energy that accounts for the physical impossibility of ‘perceptual motion machines,’ and the same principle is apparently violated by dualism.8

In light of the argument advanced in preceding section, I

claim that Dennett is wrong to suppose in this passage that

difference-making requires the transference of energy (or, for

that matter, the transference of any other physical quantity).

Even aside from the debate over the mind-body relation, cases of

double prevention (and, more generally, causation involving

negative events) demonstrate that this is not so. Fighter’s

shooting down Enemy makes a difference to the destruction of

Target, in that the latter event counterfactually depends on the

former (counterfactual dependence is a form of difference-

making), even though Fighter transfers no energy to Target. Once

this gap between difference-making and energy transference is

acknowledged, room in logical space is opened up for an

interactionist dualism that posits causal relations between

mental and physical events that violate [Closure] but comply with

the conservation laws of physics—for instance, an interactionist

dualism that holds that the actual world is relevantly like w*.9

8 Dennett (1991: 34-5).9 Something like this sort of dualist view is explored by Gibb (2013), except that Gibb assumes that double prevention is not a genuine form ofcausation.

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Along similar lines, in “What Experience Teaches,” David

Lewis writes,

if something nonphysical sometimes makes a difference to themotion of physical particles, then physics as we know it is wrong. Not just silent, not just incomplete—wrong. Either the particles are caused to change their motion without the benefit of any force, or else there is some extra force thatworks very differently from the usual four.10

Again, in light of last section’s argument, I claim that Lewis is

mistaken in this passage. Physics is silent on whether the actual

world contains ectoplasm and prevectoplasm as we have imagined

them; accordingly, physics does not entail that nonphysical minds

never make a difference to the motion of physical particles. An

interactionist dualist who holds that the actual world is

relevantly like w* should happily embrace both horns of the

dilemma that Lewis poses in his final sentence. Yes, such an

interactionist should say, prevectoplasm does exert a kind of

nonphysical force on ectoplasm when it acts to prevent ectoplasm

from interfering with the 8-ball, but surely physics is silent on

the nonphysical forces that nonphysical substances exert on one

another. And yes, this interactionist should continue, the

prevectoplasm causes the 8-ball to fall into the corner pocket

without exerting any force (physical or nonphysical) on the 8-

10 Lewis (1990: 590).

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ball, but so it goes with double prevention generally—double

prevention typically involves causation without the cause in

question exerting a force on its effect. Physics as we know it is

incomplete, not wrong, says this interactionist dualist.

Lewis’s overall philosophical views bear on my argument in

an especially interesting way. For on one hand, Lewis is of

course the great champion of counterfactual theories of

causation, and indeed has defended counterfactual theories

against opposing views partly on the basis that such opposing

views mishandle causation involving negative events, including

double prevention.11 On the other hand, Lewis is one of the

leading figures to argue against t dualism on the basis of a

causal argument for physicalism, where, as the preceding passage

suggests, the key move in this causal argument is to appeal to

physics to support [Closure].12 I suggest that this represents a

genuine tension in Lewis’s thought, at least in the sense that

counterfactual theories of causation appear to open the door to

the sort of interactionism we have been considering, which

asserts that the actual world is relevantly like w*.

5. The Causal Argument for Physicalism

11 See Lewis (2000) and (2004).12 In addition to Lewis (1990), from which the above passage is taken, see also Lewis (1966), (1972), and (1994).

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Here, via David Papineau, is a canonical formulation of the

sort of causal argument for physicalism endorsed by Dennett and

Lewis.

(P1): [Closure]: If a physical event has a sufficient cause at time t, it has a sufficient physical cause at t.(P2): All mental events have physical effects.(P3): The physical effects of mental causes are not all

causally overdetermined.(C): Mental events are identical with physical events.13

In this concluding section of the paper I show how the result

obtained in §3, that physics does not entail causal closure,

poses a problem for proponents of the causal argument, and more

specifically for attempts to justify its first premise,

[Closure]. I note in advance that my argument is not intended to

cast doubt on physicalism itself, but just on the idea that the

causal argument captures a compelling reason to be a physicalist.

Indeed, in making my case, I will sketch alternative, non-causal

arguments for physicalism, and suggest that they are more

promising than the causal argument.

The thought you find in Dennett and Lewis is that the

[Closure]-premise is justified by a deductive argument running

from physics. Supposing at this point that Dennett and Lewis are

13 Taken from Papineau (2001), but using Kim’s formulation of the causalclosure thesis. In addition to Papineau, Dennett, and Lewis, proponents of the causal argument for physicalism include Davidson (1970), Tye (1995) and (2009), Levine (2001), and Melnyk (2003).

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mistaken—physics does not entail causal closure—the question then

becomes what alternative case should be made for [Closure]. One

option I won’t seriously consider in the present paper is that

proponents of the causal argument should set physics aside and

justify [Closure] by appealing to some distinct set of

considerations, perhaps general metaphysical principles that hold

independently of the physics of our world, or perhaps the results

of some science other than physics. I don’t think such a response

is at all promising, but here I ignore it on the grounds that it,

in effect, concedes that my argument in §3 successfully shows

that [Closure] cannot be justified on the basis of what physics

tells us about the world. This by itself would be a

philosophically interesting result, and I would be more than

satisfied if the present paper were taken to establish it.

The option I will focus on instead continues appealing to

physics, but maintains that the argument from physics to

[Closure] is abductive rather than deductive. The physical truths do

not entail [Closure], it is conceded, but there is an inference to

the best explanation argument running from those truths to [Closure].

This seems to be Papineau’s position, although he is not

motivated by—he does not consider—the sort of double prevention

cases we have been working with. At any rate, I take this

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abductive response to have a good deal of prima facie appeal, and

anticipate that it will be the line taken by most proponents of

the causal argument who concede my thesis that physics does not

entail causal closure.

I am skeptical that this abductive strategy can be made to

work. To explain my grounds for skepticism, let me begin by

briefly sketching how I understand abductive reasoning generally.

Suppose you have some body of evidence, E, that is logically

consistent with various competing hypotheses, H1, H2,…, Hn, each

of which purports to explain E.14 Given this logical consistency,

E does not conclusively rule out any of these hypotheses, but the

hope is that we can identify the hypothesis that best explains E.

Now, just what makes one explanation better than another remains

a matter of dispute, but plausible candidates for explanatory

virtues include (but are not limited to) simplicity, unification,

and plausibility (measured in terms of fit with justified

background beliefs).15 Suppose that one of the competing

hypotheses, H7 say, possesses the best combination of such

14 We can suppose that a hypothesis explains E if it specifies some of the causes of those events described by E (where E is a class of propositions). We don’t need to commit ourselves to any further substantive claims about scientific explanation for present purposes.15 Lipton (2004) is an especially influential discussion of inference tothe best explanation and of just which virtues makes an explanation best. I do not here list all the virtues Lipton considers.

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explanatory virtues. Then H7 qualifies as the best explanation of

E, where this is taken as a reason to believe H7 rather than its

competitors. In this way, E lends abductive support to H7 even

though it does not deductively entail it.

In the case at hand, let the body of evidence E consist of

the class of all physical truths.16 Let the first explanatory

hypothesis to consider be [Closure], and let H* be the competing

hypothesis that the actual world is relevantly like w* as it was

described back in §3—that is, the hypothesis that all physical

truths are just as they actually are (however that is, exactly)

but there are nonphysical minds that violate [Closure] via double

prevention. What both [Closure] and H* purport to explain, then,

includes why billiard balls (or any other physical entities)

don’t decelerate in the absence of physical forces acting on

them, why the conservation principles of physics are not

violated, and so on. If proponents of the causal argument for

physicalism are to justify [Closure] on the basis of an abductive

argument from physics, what they must show is that [Closure]

provides a better explanation of E than H* does. So then, exactly

16 Alternatively, we could restrict E to some subclass of physical truths, like all physical truths presently known. My argument does not turn on our present ignorance, however, and so in order to make my point, I am happy to suppose that E includes absolutely all physical truths.

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which explanatory virtues does [Closure] possess in greater

abundance than H*?

Start by considering the virtue of simplicity. Plausibly,

other things being equal, a simpler explanation is a better

explanation, in accordance with (one understanding of) Occam’s

Razor.17 So then, is [Closure] simpler than H*? To help us think

about the issue, notice that if we grant that the causal argument

is valid (and I do grant this), [Closure] is necessarily

equivalent to—that is, holds in all the same logically possible

worlds as—the following disjunction: either (i) physicalism is

true, or (ii) physicalism is false and nonphysical mental events

are epiphenomenal with respect to physical effects (in which case

(P2) of the causal argument is false), or (iii) physicalism is

false and nonphysical mental events causally overdetermine (along

with physical causes) all their physical effects (in which case

(P3) is false). Let’s consider these disjuncts one-by-one,

proceeding in reverse order.

I deny that there is any sense in which (iii) is simpler

than H*. Each hypothesis posits a realm of nonphysical mental 17 Although for influential critical discussion of Occam’s Razor, see Sober (1981) and (1994). Here I won’t try to defend the principle that asimpler explanation is more likely to be true, but observe that any proponent of the abductive case for [Closure] who appeals to simplicity as an explanatory virtue presumably will be committed to such a principle.

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events causing physical effects. In the case of (iii) these

mental events act as overdetermining causes, while in the case of

H* they are non-overdetermining but double preventing causes. I

see no obvious difference in simplicity. I also deny that (ii) is

any simpler than H*, given that each hypothesis posits both a

physical realm and a nonphysical mental realm. Maybe you could

reply here that (ii) is simpler in that the nonphysical mental

events it posits are not causes of physical effects. But then I

would reply in turn that, first, I do not see why this should be

regarded as a mark of simplicity—the epiphenomenalist’s ontology

includes all the same nonphysical mental events that are included

in the ontology of the proponent of H*—and, second, even if it

were so regarded, this alleged advantage in simplicity that (ii)

has over H* is more than outweighed by the fact that H* but not

(ii) is consistent with our justified background belief that

mental events sometimes cause physical effects. That is, it is

outweighed by the implausibility of epiphenomenalism, where

plausibility is taken to be another explanatory virtue in

addition to simplicity.

But now consider disjunct (i), physicalism. Here I concede

the point: physicalism really is simpler than H*, in that H*

posits all the same physical entities that physicalism does and,

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in addition, posits a realm of nonphysical mental events. In a

very straightforward way, then, physicalism is the more

ontologically parsimonious view. But if this is the point that is

supposed to bear argumentative weight, it seems that the case we

are making has collapsed into a simplicity-based argument for physicalism.

There is of course a long tradition of defending physicalism on

the grounds that it is simpler than its alternatives—that is, as

opposed to defending it by appealing to any causal argument. J. J. C. Smart’s

defense of type physicalism ultimately rested on the point that

the view was simpler than dualism, while Lewis’s version of the

causal argument was partly a reply to Smart—he thought that

Smart, by focusing on simplicity, had not identified the most

compelling reasons for being a physicalist.18 More recently,

Christopher Hill, Brian McLaughlin, Ned Block and Robert

Stalnaker have all relied on simplicity considerations rather

than a causal argument to defend physicalism.19

18 Smart (1959). Lewis (1966: 17) writes that he opposes those physicalists like Smart “who say only that we are free to accept [physicalism], and should for the sake of some sort of economy or elegance.” For further critical discussion of the role simplicity plays in Smart’s case for physicalism, see Glassen (1976), and Smart’s (1978) reply. 19 Hill (1991), Hill and McLaughlin (1999), McLaughlin (2007), Block andStalnaker (1999). For critical discussion of these appeals to simplicity, see Kim (2005: Ch. 5), who thinks the causal argument is theonly promising argument to make for physicalism.

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You might think of it like this. H* is a dualist

hypothesis, and so if you are a proponent of the causal argument

for physicalism, you should seek to rule it out on the basis that

it violates some justified causal principle. As a comparison, you

rule out epiphenomenalist dualism on the basis that it violates

your premise (P2), a causal principle, and you rule out

overdeterminationist dualism on the basis that it violates (P3),

another causal principle. But according to the line of thought we

are presently developing, H* is not ruled out on the basis that

it violates any causal principle. Instead, it is ruled out on the

basis that it is insufficiently simple, a non-causal

consideration.

You might have hoped to ruled out H* on the basis that it

violates [Closure], which is a causal principle and which is the

remaining premise, (P1), of your argument. But by the lights of

the abductive case you are presently advancing, we are justified

to accept [Closure] only after we have already ruled out H* as an

inferior explanatory hypothesis. In another words, according to

your own view, it’s not that we reject H* because it violates

[Closure], it’s that we accept [Closure] only because we first

reject H* on the basis of its insufficient simplicity.

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What you set out to provide was an abductive case for

[Closure] in order to justify the first premise of your causal

argument, but in fact the case you have offered seems to collapse

into an alternative, non-causal argument for physicalism, an

argument from simplicity. Of course, if such simplicity

considerations help justify us in accepting physicalism, then by

extension they justify us in accepting [Closure], given that

physicalism entails [Closure] (as noted in §3). And so, in this

way, maybe you have provided abductive justification for

believing [Closure]. But this sort of justification is of no use

to the causal argument, at least if the causal argument is

supposed to be our basis for initially accepting physicalism.

What the causal argument needs is some justification for

[Closure] that does not depend on some prior, independent

justification for physicalism provided by some alternative

argument.

The present problem generalizes to other explanatory virtues

beyond simplicity. Take unification. It is plausible that, all

else equal, a better explanation unifies a wide range of

phenomena in the sense that it shows them to be connected, with

the stock examples being Newton’s unification of celestial and

terrestrial mechanics and Maxwell’s unification of electricity

24

and magnetism.20 Again, I deny that (ii) or (iii) achieve greater

unification than does H*. In fact, each of these three hypotheses

posits a fundamentally disunified world in the sense that they

claim there are nonphysical mental phenomena that are wholly

different from physical phenomena.

On the other hand, the physicalist disjunct (i) does achieve

greater unification. It unifies chemical, biological, mental, and

all other phenomena in that it proposes that they are all

ultimately physical (or physically realized), governed by the

same physical laws, and so on. But again, if this is the point

that is supposed to bear weight, it seems like the abductive

argument we should be making runs from physics not to [Closure],

but directly to physicalism itself. Once physicalism is

abductively established in this way, we can go back and deduce

[Closure] as an afterthought, but our justification for accepting

[Closure] then depends on our prior justification for accepting

physicalism.

I have been framing things in terms of the priority of

justification between [Closure] and physicalism. Alternatively,

it might be clarifying to frame things in terms of what we should

20 Friedman (1974) and Kitcher (1981) and (1989) advance unificationist accounts of scientific explanation, but you don’t need to accept such accounts to regard unification as an explanatory virtue.

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infer if we were to acquire a defeater for physicalism. Suppose,

for instance, that we have been persuaded by Frank Jackson’s

Mary’s room argument, or David Chalmers’s zombie argument.21 It is

widely thought that we will then be forced into an

epiphenomenalist dualism—Jackson explicitly endorses

epiphenomenalism, while Chalmers is at least open to the

suggestion.

But embracing epiphenomenalism in this scenario makes sense

only if we possess justification for believing [Closure] that

does not depend on prior justification for physicalism itself. If

our justification for [Closure] does so depend on our prior

justification for physicalism, then if we were to acquire a

defeater for physicalism—in the form of a compelling argument for

dualism—we would be left without a justification for [Closure],

and so without any reason to prefer epiphenomenalism to an

interactionist dualism like H* that preserves mental causation by

positing violations of [Closure]. In short, the dualist foe that

physicalists should take most seriously is not epiphenomenalism

but the sort of interactionism of H* which violates [Closure]

while leaving all physical truths in place.

21 Jackson (1982), Chalmers (1996).

26

Stated most generally, the challenge for those proponents of

the causal argument who hope to make an abductive case for

[Closure] is that they must show not just that [Closure] is a

better explanation than H* is—this is the challenge we originally

posed to them—but also that it is a better explanation than

physicalism itself, taken as an explanatory hypothesis in its own

right. If what best explains the physical truths is physicalism,

then what we have is not an abductive case for [Closure] that we

can feed into the causal argument, but an alternative to the

causal argument—an abductive argument for physicalism. Of course,

if all we want is some compelling argument or another for

physicalism, this may be good enough. But we should be clear in

that case that the reasons for accepting physicalism are not

captured by the causal argument—the causal argument reverses the

justificatory priority between physicalism and [Closure].

Here is one way that proponents of the causal argument might

hope to meet this challenge. Physicalism entails [Closure], and

so is logically stronger than it. In connection, [Closure] is

consistent with (both epiphenomenalist and overdeterminationist)

dualism while physicalism of course is not. Now, I assume that

such consistency is no advantage if we take dualism to be an

utterly implausible view. In general, consistency with utterly

27

implausible propositions is no explanatory virtue. But suppose we

concede that there is at least some force to certain dualist

arguments, perhaps those advanced by Jackson and Chalmers. We

acknowledge that they provide some reason to accept dualism,

while leaving it open that there may be greater reason to accept

physicalism. In that case, the consistency of [Closure] and

dualism really would count as an abductive advantage of [Closure]

over physicalism. It would show a respect in which [Closure] is

more plausible than physicalism, where plausibility is again

understood as an explanatory virtue. In generally, a logically

weaker hypothesis is always at least as plausible as a stronger

hypothesis (since there is no way for the stronger hypothesis to

be true while the weaker hypothesis is false), and often the

weaker hypothesis will be more plausible.

However, the opening this provides to proponents of the

causal argument is extremely narrow. They need the case for

dualism to be so powerful that it counts as a substantial

explanatory advantage of [Closure] that it is consistent with

dualism while physicalism is not, an advantage that outweighs the

various explanatory virtues possessed by physicalism, making

[Closure] a better explanation. But at the same time, they need

the case for dualism not to be so powerful that it is stronger than

28

the causal argument for physicalism that they themselves are in

the process of advancing. Given that their aim is to feed the

abductive case for [Closure] into the causal argument for

physicalism, it would be a pyrrhic victory if they showed that

what makes [Closure] a better explanation is that physicalism is

highly implausible.

Proponents of the abductive argument for [Closure] thus must

steer a course between Scylla, represented by the hypothesis of

physicalism, and Charybdis, represented by dualist hypotheses

like H*. They need to pinpoint explanatory advantages that

[Closure] has over H* that don’t simply collapse into advantages

for physicalism (such as consistency and unification, perhaps),

and at the same time pinpoint explanatory advantages that

[Closure] has over physicalism that don’t simply collapse into

advantages for dualist hypotheses like H* (such as consistency

with dualism).

I have no knockdown argument that this can’t be done, in

part because I have no comprehensive list of the explanatory

virtues or an account of how to weigh different virtues against

one another. However, I hope I have made it clear that there is a

substantial burden on proponents of the causal argument if this

is their response to my thesis in this paper, that physics does

29

not entail causal closure. It’s not enough just to gesture

vaguely that some sort of abductive case can be made for

[Closure]. You must work out just how such an abductive case is

supposed to go, and be sensitive to the threat that it will

collapse into an abductive case for physicalism itself.

Given this result, you can see how much tidier things would

have been for proponents of the causal argument if only physics

had entailed causal closure. In that case, the charge they could

make against interactionist dualists rejecting [Closure] is that

such dualists must reject physics; they must hold that physics is

wrong rather than just silent or incomplete, as Lewis puts it.

This would allow proponents of the causal argument to bypass all

the complications raised in this final section regarding an

abductive case for [Closure]. But, alas: physics does not entail

causal closure.

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