Explaining Causal Closure
Explaining Causal Closure
Justin Tiehen
The physical realm is causally closed, according to physicalists like me. But why
is it causally closed? What explains causal closure? In what follows I argue that
physicalists are committed to one explanation of causal closure to the exclusion of others,
and that this means they must give up on using a causal argument to defend their view.
The problem with dualism is not causal in any interesting sense.
1.
Some Potential Explanations
Following Jaegwon Kim, the causal closure thesis can be formulated as follows.
(Closure): If a physical event has a cause at a time t, it has a physical cause at t.1
This formulation makes transparent the logical relation between causal closure and the
core physicalist thesis, that everything is physical. Or, limiting our attention to events,
(P*): Every event is physical.
I will treat (P*) as a commitment of physicalism.2 If every event is physical, it trivially
follows that whenever some physical event has a cause, that cause will be physical: (P*)
entails (Closure). This entailment corresponds to the first proposal for explaining causal
closure, which says that the physical realm is causally closed just because every event is
physical. (Closure) is true because (P*) is.3
Consider an analogy. There are no unicorns. Taking a non-unicorn event to be one
whose constitutive object is not a unicorn, the nonexistence of unicorns entails,
1
Kim (2005: 15).
Some nonreductive physicalists deny (P*) and take mental events to be realized by, not identical with,
physical events. Their view—that every event is physical or physically realized—also entails (Closure), but
to ease exposition I will ignore it here and suppose all physicalists are token physicalists who accept (P*).
3
Here and throughout I operate with an “ontic” conception according to which facts or events in the world
explain other facts or events. I don’t distinguish between facts and true propositions, but nothing I say rules
out a view that draws such a distinction.
2
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Explaining Causal Closure
(U-Closure): If an event has a cause at a time t, it has a non-unicorn cause at t.
(U-Closure) is true. Why is it true? It’s true because there are no unicorns. Intuitively,
that’s the explanation. The present proposal says that the explanation of physical causal
closure is exactly like this.
Alternative explanations of physical causal closure are imaginable. Maybe the
physical realm is causally closed because there is something about physical events—
something almost like a shield surrounding them—that prevents impingements from the
nonphysical realm, or at least that permits such impingements only when a physical cause
is also taking place. This thought can be developed in terms of laws of nature. Maybe
(Closure) is true because it is a law.4 In putting things this way I assume laws explains
their instances: (Closure)’s lawhood would explain its mere truth. Alternatively one
might say that if (Closure) is a law it is explanatorily basic and so its truth has no
explanation. I won’t be putting things this way, but I have no deep objection to doing so.
What matters is that we keep track of the difference between this second proposal and the
first, where (Closure)’s truth is explained by (P*)’s.
Yet further explanations seem imaginable as well, but for our purposes we can
stop at these two. Replace the second explanation just offered with any third you please
and all the arguments offered below still go through.
2.
Explanatory Independence
Physicalists, I claim, are committed to accepting the first explanation of causal
closure to the exclusion of the second, and more generally to the exclusion of any
alternative explanation. The first step in defending an explanatory exclusion claim of this
4
I assume here that a universal generalization like (Closure) is the sort of thing that could be a law. But
everything I say could be made compatible with alternative conceptions of laws, like the
Tooley/Dretske/Armstrong view on which laws are relations among universals.
2
Explaining Causal Closure
sort is establishing that the explanations in question are independent of one another. That
is what I will do in this section.
Imagine God is building a world and the one thing he wants to guarantee about
the place is (Closure)’s truth there. He has some options. First, he could decree that no
nonphysical events are to take place—that is, that (P*) is to be true. If God did this he
wouldn’t also need to go to the trouble of making (Closure) a law, since (P*)’s truth
ensures (Closure)’s truth regardless of (Closure)’s lawhood. Second, he could decree that
causal closure is a law at his world. If God did this he wouldn’t also need to go to the
trouble of making (P*) true, since (Closure)’s lawhood ensures its truth regardless of
whether (P*) is true. Third, God could do both these things. He could both decree (P*) is
true and also issue a separate decree that (Closure) is a law. If these are three distinct
ways God could build his world, then the two proposed explanations of (Closure)’s truth
are independent of one another.
Counterfactuals give us another, less metaphorical way of drawing out the
independence. Consider a principle of the form: if P explains Q, then (barring
overdetermination, preemption, etc.) if P had not obtained, Q would not have obtained.
Some such principle is widely accepted in the special case of causal explanation, but here
I intend it as a principle governing explanation generally. Applying the principle, the
counterfactual to consider is this: if there had been nonphysical events, the physical realm
would not have been causally closed. That is,
(CF): ~(P*) > ~(Closure).
The truth value of (CF) depends on what explains (Closure). If the physical realm
is causally closed only because there are no nonphysical events, it follows that if,
3
Explaining Causal Closure
counterfactually, there had been such events, the physical realm wouldn’t have been
causally closed. That is, if (Closure) is true just because (P*) is, (CF) will be true. In
contrast, if causal closure is a law of nature, it follows that if, counterfactually, there had
been nonphysical events, the physical realm still would have been causally closed. Laws
support counterfactuals, and so if (Closure) is a law it figures to obtain in the closest
~(P*) worlds. (CF) will then be false.
Consider then three different worlds. At the first, the physical realm is causally
closed only because every event is physical and not because causal closure is a law.
There, both (P*) and (CF) are true. At the second, the physical realm is causally closed
only because it is a law and not because every event is physical. There, both (P*) and
(CF) are false. At the third, the physical realm is causally closed both because it is a law
and also because every event is physical. There, (P*) will be true while (CF) is false. If
these three worlds with their different assignments of truth values are all possible, the two
proposed explanations of causal closure must be independent of one another.
3.
Explanatory Exclusion
Given this independence, I say the two explanations exclude one another.
Reconsider the third divine building plan. Given that the goal is merely to ensure
(Closure)’s truth, a God who accomplishes this by both making (P*) true and also making
(Closure) a law has done double the work needed. He has made (Closure)’s truth
overdetermined, in a sense. This is not causal overdetermination; (Closure)’s lawhood
does not cause its truth, for instance. Rather it is a matter of there being two independent
facts—that all events are physical, and that causal closure is a law of nature—each of
which by itself guarantees (Closure)’s truth. Each of which explains its truth.
4
Explaining Causal Closure
Call this explanatory overdetermination. Causation is one type of metaphysical
determination relation and so causal overdetermination is a special case of explanatory
overdetermination. There are various other metaphysical determination relations as well
though: realization, supervenience, constitution, composition, and so on. Wherever there
is metaphysical determination there is the potential for overdetermination, or at least for a
view entailing such overdetermination. This is an idea I take from Kim, in his most recent
presentations of his exclusion argument.5 To bring out the parallels between causal and
non-causal explanatory overdetermination, let us turn to the causal argument for
physicalism, which will be our focus in the remainder of the paper.
Surely there is something objectionable about a mind-body theory positing
systematic causal overdetermination in order to account for mental causation. This
thought plays a central role in the causal argument, which I formulate as follows.
(P1): If a physical event has a cause at a time t, it has a 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.6
Observe that the first premise just is (Closure), while the conclusion I will be treating as
equivalent to our (P*).7 For now, focus on the anti-overdetermination premise (P3). Why
accept it? The issue is controversial, but here is one stab at an answer.
5
Kim (2005: 39-40) suggests there is a prima facie tension between holding that a mental state M both
supervenes on a physical state P and is caused by some distinct mental state M’. Given that supervenience
and causation are both determination relations, M seems overdetermined, though of course not causally
overdetermined since supervenience is not a causal relation. According to Kim the way to resolve this
tension is by supposing that M’ causes P. This, however, leads to the familiar causal exclusion problem,
since P, being a physical state, has fully sufficient physical causes.
6
This version is taken from Papineau (2002) with minor adjustments. Besides Papineau, proponents of the
causal argument include Smart (1959), Lewis (1966), Davidson (1970), Tye (1995) and (2009), Levine
(2001), and Melnyk (2003). It is the single most influential argument offered for physicalism today.
7
(C) says that all mental events are physical, while (P*) says that absolutely all events are physical. These
claims are equivalent if we assume the via negativa approach, which defines “the physical” as the nonmental. Even if we do not assume this, treating (C) and (P*) as equivalent is harmless since proponents
typically take the causal argument to generalize to all events.
5
Explaining Causal Closure
I assume the problem is not that causal overdetermination is metaphysically
impossible. Presumably there are bizarre worlds out there where (P3) is false, just as
there are worlds where everyone who dies is killed by a pair of simultaneous gunshots to
the heart. Instead the problem seems epistemic in a way. It is difficult to imagine what
evidence could reasonably convince us we live in a world where (P3) is false. To bring
out the difficulty, imagine we already possess fully sufficient causal explanations for all
physical effects in terms of purely physical causes. What warrant could we then obtain
for positing additional nonphysical causes of those same physical effects? Doing so
inevitably would seem gratuitous; it would bring no gain in explanatory power.
The same dynamic arises in our case of non-causal overdetermination. Taking
(Closure)’s truth to be explanatorily overdetermined is objectionable, but not because
such overdetermination is metaphysically impossible. I assume there are worlds where it
is both the case that (P*) is true and also that (Closure) is a law. The trouble is that it is
difficult to imagine what evidence could reasonably convince us that we live in such a
world. Mirroring our discussion of the causal case, imagine we already know (P*) is true
and now want to figure out whether (Closure) is a law. What evidence should we look
for? Clearly, it will not do to go out and observe various causal chains that all comply
with (Closure) while never finding a single chain that violates it. Such observed
compliance would fail to generate pressure on us to infer (Closure)’s lawhood, since we
already possess a fully sufficient explanation for such compliance: the truth of (P*).
Given (P*)’s truth, we know in advance that all causal chains will comply with (Closure)
regardless of whether or not it’s a law. But in that case, observed compliance could do
nothing to confirm (Closure)’s lawhood.
6
Explaining Causal Closure
Admittedly, there also will not be any disconfirming evidence of (Closure)’s
lawhood in the form of causal chains violating (Closure). There can’t be any such chains,
given (P*)’s truth. There is an asymmetry between lawhood and non-lawhood, however.
For familiar reasons of parsimony, a theory should posit as few explainers as it needs to
get by. Laws are explainers, and so there is an initial presumption against the lawhood of
any given proposition, a presumption that can be overridden only when a gain in
explanatory power is to be had. Given (P*)’s truth, all the explanatory work that a
(Closure) law might hope to do is already done, leaving such a law explanatorily idle. It
is thus explanatorily excluded.
This at least is my preferred analysis of what’s wrong with explanatory
overdetermination, both causal and non-causal.8 For the sake of the arguments that follow
it is not strictly required that you accept the analysis. It is good enough if you agree that
non-causal overdetermination is problematic in the same way that causal
overdetermination is, whatever way that happens to be.9 This has a good deal of prima
facie plausibility even aside from the analysis offered here. Any alternative account of
what’s wrong with causal overdetermination that I can think of seems to generalize to
metaphysical determination relations generally.
What if you think that explanatory overdetermination in all its forms is
unproblematic?10 Then even prior to my objections in this paper, you should reject the
8
The analysis entails that explanatory overdetermination in general is problematic just when there are not
independent sources of evidence for the different explainers. If a pair of simultaneous shots to the heart
leave separate entrance wounds, this need not be an objectionable case of overdetermination—there is
separate evidence for the efficacy of each shot. This is very different from the sort of overdetermination at
issue in (P3), since mental and physical causes do not leave separate traces. Compare Kim (2005: 48) on
this very point.
9
Cf. Kim (2005: 48).
10
See for instance Crisp and Warfield (2001), Sider (2003), and Schaffer (2003), who defend causal
overdetermination.
7
Explaining Causal Closure
causal argument for physicalism, given its anti-overdetermination premise (P3). You can
think of it as a dilemma for the causal argument. Either positing explanatory
overdetermination is objectionable or it is not. If not, the causal argument is in trouble
given its anti-overdetermination premise (P3). On the other hand, if it is objectionable
then the causal argument is in trouble because, as I will show in the remainder of the
paper, it requires the explanatory overdetermination of (Closure)’s truth. Either way then
the causal argument for physicalism is in trouble.
4.
The Intuitive Idea to the Causal Argument
Focusing on the second horn of this dilemma, let’s assume in what follows that
explanatory overdetermination is something to avoid. Then, since physicalists must
accept (P*), and since (P*)’s truth sufficiently explains (Closure)’s, it follows that no
physicalist can accept any distinct explanation of (Closure)’s truth, on pain of positing
overdetermination. This result does not undermine physicalism itself—a point I will
return to below—but it is fatal for the causal argument. My discussion proceeds in two
parts. In this section, I show that the present point undermines the intuitive idea driving
the causal argument, the idea that dualism is problematic in a distinctively causal way. In
the sections that follow I will go beyond this intuitive assessment and pinpoint a specific
epistemic flaw in the causal argument that arises as a result, a flaw regarding the
transmission of warrant.
Starting with the intuitive point, philosophers often treat physical causal closure
as a deep truth about the world, but the fact is that true causal closure theses are a dime a
dozen. You get one whenever you have a nonexistent. We saw this above with unicorns
8
Explaining Causal Closure
but here let’s vary examples. The Kansas City Royals baseball team did not win the 2003
World Series, and so the 2003 Champion Royals do not exist. This entails,
(KC-Closure) If an event has a cause at a time t, it has a non-Champion Royal
cause at t.11
We can feed this causal closure thesis into a causal argument against the Royals.
(P1): (KC-Closure).
(P2): The 2003 World Series Champions are not epiphenomenal.
(P3): The 2003 World Series Champions are not causally redundant.
(C): The Kansas City Royals did not win the 2003 World Series.
The analogy to the causal argument for physicalism is obvious. Each argument
consists of a causal closure premise, an anti-epiphenomenalism premise, and an antioverdetermination (or causal redundancy) premise. The causal argument for physicalism
purports to establish something about the nature of mental events, namely that they are
not nonphysical. The causal argument against the Royals purports to establish something
about the nature of the 2003 World Series winners, namely that they are not the Royals.
The causal argument against the Royals is sound: each premise is true and its
conclusion deductively follows. Indeed, its soundness is far less controversial than that of
the causal argument for physicalism—there really are epiphenomenalists about the
mental, but not (to my knowledge) about the 2003 World Series winners. Still, something
about this causal argument against the Royals is absurd. The hypothesis that the Royals
won the 2003 World Series is false, but it is not objectionable in a distinctively causal
way—not in the way that, say, platonism in mathematics is often taken to be specifically
causally objectionable. Next section I will diagnose exactly what is wrong with the causal
argument against the Royals; here, I just want to draw out why it seems absurd.
11
A non-Champion Royal event is one whose constitutive object is not a member of the 2003 World Series
Champion Kansas City Royals.
9
Explaining Causal Closure
We take a step in the right direction by reflecting on counterfactuals. Yes, the
2003 Champion Royals fail to cause anything. But that is only because they don’t exist!
If the Royals had won the 2003 World Series, they would have caused all sorts of things
and (KC-Closure) would have been violated left and right. The following counterfactual,
an analogue to (CF), is plainly true.
(KC-CF): The Royals won the 2003 World Series > ~(KC-Closure).
The truth of (KC-CF) shows that what the Champion Royals really suffer from is a
general existence problem, not a distinctively causal problem. If only they possessed a
solution to their existence problem—by existing—there would be no remaining causal
obstacle for them to overcome.
More generally, absolutely everything is such that it would not enter into causal
relations if it did not exist.12 Unless we want to trivialize what it is for a hypothesis to
count as causally problematic, we should reserve this description just for those
hypotheses positing entities that would be epiphenomenal or causally redundant even if
they existed. Perhaps platonic numbers are causally problematic in this sense, but surely
the 2003 Champion Royals are not. And this, I suggest, is why no one tries to run a
causal argument against the Royals, the truth of (KC-Closure) and the soundness of the
causal argument against them notwithstanding.
(KC-CF)’s truth, which again we took to show that the Royals have an existence
problem rather than a distinctively causal problem, follows from what explains (KCClosure)’s truth: that the 2003 Champion Royals do not exist. Taking the case as
illustrative, I propose that a nonexistent is not distinctively causally problematic
12
This applies even to absences: either they are causes and thus exist, or they do not exist and thus are not
causes. Further discussion would take us too far afield.
10
Explaining Causal Closure
whenever its corresponding causal closure thesis is true only because the entity does not
exist. Consider unicorns, or the milk in the fridge (which I finished yesterday), or the
John McCain presidency. In each case there is a true causal closure thesis corresponding
to these nonexistents, and so a sound causal argument to be made against them.
Nevertheless, intuitively none of these entities is distinctively causally problematic since
in each case the given causal closure thesis is true only because the entities in question do
not exist.
You may be tempted here to say that nonexistence is a kind of causal problem,
and so there is a sense in which these nonexistents are causally problematic after all.
Fine: say this. But then appreciate that nonexistents have a number of parallel problems
having nothing to do with causation. The nonexistence of the 2003 Champion Royals
entails not just (KC-Closure) but also (KC-Time): Everything that begins to exist is not a
member of the 2003 Champion Royals; and (KC-Identity): Everything that is self
identical is not a member of the 2003 Champion Royals; and so on. The 2003 Champion
Royals have a causal problem only in the same attenuated sense that they have a temporal
problem (uncontroversial premise: the 2003 World Series Champions began to exist at
some point), a self identity problem (uncontroversial premise: the 2003 World Series
Champions are self identical), and so on. The Royals’ problem is not distinctively causal.
The same is true of nonphysical mental events. What they have is a general
existence problem, not a distinctively causal problem. Or, at least this is what you must
hold if you are a physicalist. For again, physicalists are committed to holding that every
event is physical, (P*), which by itself sufficiently explains why the physical realm is
causally closed, (Closure). For physicalists to posit some additional explanation of
11
Explaining Causal Closure
(Closure)’s truth would be for them to embrace explanatory overdetermination, which we
are assuming (in accordance with the causal argument) must be avoided. But, if (Closure)
is true only because (P*) is, it follows that if, counterfactually, there had been
nonphysical events, the physical realm would not have been causally closed. That is,
(CF)’s truth follows. In that case, nonphysical mental events are causally no different
than the 2003 Champion Royals or unicorns or the milk in the fridge or the McCain
presidency. Again, if you want to say this is a kind of causal problem for dualism, go
ahead. But then appreciate that dualism has a causal problem only in the same attenuated
sense that it has a temporal problem (everything that begins to exist is physical), a self
identity problem (everything self identical is physical), and myriad other non-causal
problems as well. Dualism’s problem is not distinctively causal.
This completely undermines the thought behind the causal argument. Here is
David Papineau offering a characteristic expression of that thought.
If conscious properties were non-material, they would thus be epiphenomenal
‘danglers’, caused by physical occurrences but themselves having no effects on
physical activities . . . if there were compelling independent grounds for holding
that conscious properties are non-material, then we would have no option but to
accept epiphenomenalism about consciousness.13
Right: this is supposed to be dualists’ causal problem. The trouble is that Papineau is
guilty of overdeterminationist thinking. As a physicalist, he holds that everything is
physical, which by itself guarantees physical causal closure. But in addition, he assumes
in the passage that there is some further, unspecified, overdetermining reason the physical
realm is causally closed, a reason that would hold even under his counterfactual
supposition that conscious properties were non-material. Physicalists who reject
overdeterminationist thinking should reject what Papineau says here. Just as no one
13
Papineau (2001: 11).
12
Explaining Causal Closure
thinks that the 2003 World Series Champions would be epiphenomenal danglers if the
Royals had won, no physicalist should hold that conscious properties would be
epiphenomenal danglers if they were nonphysical.
Metaphysically, what would suit the purposes of Papineau and others who accept
the intuitive idea of the causal argument is if causal closure were a law of nature that held
independently of physicalism itself. Such a (Closure) law would hold even if (P*) were
false, since again laws support counterfactuals. In that case, dualism really would have a
distinctively causal problem in a way that ordinary nonexistents like the Champion
Royals do not: even if they existed, nonphysical events could not (non-redundantly)
cause physical events. Unfortunately for proponents of the causal argument, physicalists
seeking to avoid explanatory overdetermination cannot believe in such a (Closure) law.
This is not a modal point. Suppose with Kripke that unicorns are metaphysically
impossible.14 This entails the metaphysical necessity of (U-Closure). Still, even if
violations of (U-Closure) are thus impossible, that does not make unicorns distinctively
causally problematic, for it continues to be the case that what explains (U-Closure)’s
truth/necessity is just the truth/necessity of the proposition that there are no unicorns.
Given that this is the explanation, if—per impossibile!—there were unicorns, they would
violate (U-Closure) left and right.15
In connection, in claiming here that physicalists cannot accept that (Closure) is an
independent law of nature, I am not saying that physicalists must deny its modal
robustness. Perhaps (Closure) is a metaphysical or nomological necessity because (P*) is
14
Kripke (1972: 24).
This is a counterpossible, a counterfactual with an impossible antecedent. I assume without argument that
counterpossibles can be non-vacuously true, although this is at odds with the standard Lewis-Stalnaker
analysis of counterfactuals. On the use of counterpossibles to draw out the consequences of necessarily
false metaphysical theses, see for instance Sider (1999: 339-340) and Merricks (2003: 5-8).
15
13
Explaining Causal Closure
a metaphysical or nomological necessity. Regardless, the point is that physicalists must
deny that (Closure)’s truth has some explanation independent from (P*)’s truth, which
means they must deny that (Closure) would hold if (P*) did not. If this is right, then
nonphysical mental events are no more causally problematic than unicorns are, regardless
of whatever modal strength we wish to assign to (Closure) or (U-Closure).16
Also, do not think of this as a point about what follows from physics. Defenders
of the causal argument often claim that (Closure) is entailed by physics, our leading
science. I am sure this is right. Indeed, as a physicalist I hold that the physical truths
entail all truths, and so a fortiori they entail all true causal closure principles—(UClosure) and (KC-Closure) just as much as (Closure). But this does not make nonphysical
mental events distinctively causally problematic any more than it makes unicorns or the
Champion Royals distinctively causally problematic. Again, what matters is what
explains the truth of these causal closure principles.
The conclusions reached in this section undermine the intuitive idea behind the
causal argument for physicalism, but do not by themselves provide a diagnosis of exactly
where the argument goes wrong. That will be provided in the next section.
5.
Cogency
It will be helpful to draw on recent epistemological work on the transmission of
warrant.17 To know a conclusion on the basis of a particular argument, your belief in the
argument’s premises must be warranted—otherwise, your subsequent belief in the
conclusion will not rise to the level of knowledge—and your warrant for those premises
If (P*) is a law and the laws are closed under deductive entailment—which they aren’t, but pretend—
(Closure) will count as a law too. But it won’t be a law that holds independently of physicalism itself,
which is the crucial issue.
17
See for instance Wright (2003) and (2004), Davies (2004), McLaughlin (2003), Pryor (2004), Silins
(2005), and Tucker (2011).
16
14
Explaining Causal Closure
must not essentially depend on prior warrant for the conclusion itself—otherwise, the
basis of your knowledge of the conclusion will be the source of that prior warrant and not
the argument in question. Following Crispin Wright, let’s say that arguments not meeting
these conditions are not cogent.18 I claim that although the causal argument for
physicalism is sound, it is not cogent in this sense.
To get a feel for what this amounts to, reconsider the causal argument against the
Royals. Again, the argument is undeniably sound. Still, I suggest it is defective in that the
only available warrants we have for its (KC-Closure) premise depend on antecedent
warrants for its conclusion. In effect, we know (KC-Closure) is true only because we first
know that the Royals did not win. This seems right: it was no accident that I introduced
(KC-Closure) by observing that it follows from the fact that the Royals did not win. The
causal argument for physicalism is similarly uncogent, I say. We know the physical realm
is causally closed, but we know this only because we first know the truth of physicalism.
Discussions of cogency and warrant transmission often focus on arguments
alleged to be question begging, like Moore’s proof of the external world. But an
argument can fail to be cogent in the sense we have specified without any question being
begged.19 Imagine I am advancing the causal argument against the Royals, and to support
its (KC-Closure) premise I provide videotape evidence of the 2003 World Series showing
that the Royals did not win (they did not even participate). This evidence really does
warrant belief in (KC-Closure), and it does so in a non-question begging way. Even if
you initially doubted (KC-Closure) or that the Royals did not win, the video evidence
should overcome your doubts. But the resulting causal argument against the Royals is
18
19
Wright (2003).
Cf. Pryor (2004) and Tucker (2011).
15
Explaining Causal Closure
defective in that anyone possessing this warrant for its (KC-Closure) premise is already
warranted to believe its conclusion even without running through the argument. If you
have the video proof, you no longer need the causal argument.
Lack of cogency is a general problem for causal arguments. Again, for any
nonexistent there is a corresponding true causal closure thesis which can be fed into a
sound causal argument against the existence of that entity. Why aren’t causal arguments
much more common then, why aren’t they deployed all over the place? Because they are
almost never cogent. Generally, there is no realistic way of knowing the truth of a given
causal closure thesis short of first knowing that its corresponding entity does not exist.
Given that this is the general state of things, it is unsurprising that the causal argument for
physicalism should fail to be cogent as well.
6.
The Inductive Case for Causal Closure
This charge of uncogency against the causal argument for physicalism can be
rebutted if its defenders come up with a warrant for (Closure) that does not depend on
any prior warrant for physicalism itself. In principle, an a priori defense could be
mounted,20 but this option attracts few contemporary physicalists and would drain the
case for physicalism of any serious empirical content.21 For many physicalists, myself
included, the leading attraction of physicalism is its supposed empirical superiority to
dualism, and so I will assume that what is sought is an empirical warrant for (Closure).
Here is Andrew Melnyk, making the sort of case needed.
Davidson’s (1970) and (1995) case for (Closure) looks pretty a priori. The “pairing problem” discussed
by Foster (1996: Ch. 6) and Kim (2005: Ch. 3) also potentially serves as the basis for an a priori defense.
In attacking his interactionist dualism, a number of Descartes’ critics through the years have made use of
seemingly a priori arguments for (Closure).
21
The second and third premises of the causal argument are supported by armchair considerations. If the
causal argument is to rely on any empirical support, it must be support for its (Closure) premise.
20
16
Explaining Causal Closure
[It is not true] that in order to be persuaded of the causal closure of the physical
one must already be persuaded of physicalism. To see this, it is necessary only to
review how the closure principle is usually evidenced. First we become
persuaded, on the basis of observational evidence and ordinary canons of
scientific reasoning, that various physical effects have sufficient physical causes,
since the best available explanations of those effects posit physical and only
physical causes; surely no assumption of physicalism is needed to take the first
step. Then, employing enumerative induction, we treat these well-supported
explanations as evidence that all physical effects have sufficient physical
causes.22
I really do think that causal argument proponents need something like this
inductive defense. Sometimes philosophers try to capture dualism’s alleged causal
problem by imagining taking some physical effect putatively caused by a mental state,
like crying putatively caused by pain, and tracing back the causal chain leading up to the
effect, finding physical causes each step of the way. This is perfectly fine as a way of
picturing things, but don’t mistake it for a line of argument distinct from Melnyk’s
inductive case. We can just as easily imagine tracing the causal chain back and finding
that nonphysical ectoplasm occupies the pain causal role. Why favor one act of the
imagination over the other? Because, a defender of the causal argument should say, there
is an impressive inductive case for (Closure). I thus will make the inductive defense of
(Closure) my focus in what follows. At the very least, my response to the inductive case
will provide a template for responding to other defenses of (Closure), in case there are
viable alternative defenses I am overlooking.
What I will argue is that no physicalist can accept that (Closure) is susceptible to
inductive confirmation. In broad outline, the idea is that there are close connections
between inductive confirmation, counterfactuals, and explanation. Because of these
connections, accepting an inductive case for a hypothesis inevitably involves taking on
22
Melnyk (2003: 289-290). See also Papineau (2002: Ch. 1) for an influential inductive defense.
17
Explaining Causal Closure
certain commitments regarding counterfactuals, which in turn involves taking on certain
commitments regarding what explains what. In the case that interests us, accepting an
inductive case for (Closure) commits one to holding that (Closure)’s truth has some
explanation besides just (P*)’s truth. But as we have already seen, no physicalist can
embrace this explanatory commitment. It follows that no physicalist can accept (Closure)
on the basis of an inductive case.
Let’s begin with the widely recognized link between induction and
counterfactuals. Why is there such a link? A plausible thought is that it is because
unobserved actual instances of a hypothesis are relevantly like nearby counterfactual
instances, and so evidence E inductively warrants the belief that hypothesis H is actually
true—has no unobserved actual counterinstances—only if E also warrants the belief that
H is true in nearby counterfactual worlds—has no nearby counterfactual
counterinstances. Inductive confirmation is incapable of limiting itself just to the actual
world; inevitably, it spills over to nearby worlds.
Because of this, accepting an inductive case for a hypothesis commits you to
accepting a range of counterfactuals involving that hypothesis. Suppose I am testing
various samples of copper and find that each conducts electricity. This evidence
inductively warrants me to believe that all copper is conductive, we can suppose, but only
because it also warrants me to believe that my samples would continue being conductive
in various counterfactual scenarios. The temperature of my samples could have been
slightly warmer, and no doubt there are some actual samples that are slightly warmer. In
light of this, accepting the inductive case commits me to accepting the counterfactual that
my samples would have conducted electricity had they been warmer. If I deny this, if I
18
Explaining Causal Closure
hold instead that my samples would not have been conductive if they were warmer, this
undermines my inductive inference from my samples to the conclusion that all copper is
conductive. After all, if my samples wouldn’t have been conductive had they been
warmer, what warrant do I have to believe that those actual pieces of copper that are in
fact warmer are conductive?
Similarly, accepting an inductive case for (Closure) brings with it a commitment
to accepting a range of counterfactuals involving (Closure). Reject those counterfactuals
and you are no longer entitled to accept the inductive case. Now, if the inductive case is
to warrant (Closure) independently of any prior warrant for (P*), as it must to rebut the
charge of uncogency, it should continue to warrant (Closure) even as we counterfactually
suppose that ~(P*). This means, in effect, it should warrant belief in the counterfactual
~(P*) > (Closure). But for reasons we have seen, no physicalist can accept this
counterfactual commitment of the inductive case. For, turning to the link between
counterfactuals and explanation, physicalists must hold that (Closure) is true just because
(P*) is, and so they must accept a different counterfactual, (CF): ~(P*) > ~(Closure). If
this is right, no physicalist can accept (Closure) on the basis of the inductive case.
Here is another way to make the point. Could a defender of the inductive case
both grant that if there were nonphysical events they would violate causal closure, and
also at the same time insist that thanks to induction we know that causal closure obtains
regardless of whether or not there are nonphysical events? Surely this is incoherent. If it
is agreed that nonphysical events would violate (Closure), then how could induction
allow us to know that (Closure) is true while leaving open that there may still be such
events? The incoherence can be avoided by going back and rejecting the granted
19
Explaining Causal Closure
counterfactual, maintaining instead that the physical realm would be causally closed even
if there were nonphysical events. But in that case, the defender of the inductive case has
embraced a counterfactual that physicalists must reject if they are to avoid positing
explanatory overdetermination.
If you arrived at your physicalism on the basis of the causal argument taken in
conjunction with the inductive case for (Closure), you are now obligated to go back and
reject that inductive case, since it is at odds with your (assumed) rejection of explanatory
overdetermination. Unless you possess some other warrant for believing in physicalism,
your physicalism is now unwarranted. This is a strong conclusion. I have two additional
arguments to support it. The first adopts an offensive stance and aims to place a burden
on those who would resist this section’s objection to the inductive case for (Closure). The
second is defensive and aims to show that my objection to the inductive case does not
bear as severe a burden as it might initially seem.
7.
The Inductive Case for (KC-Closure)
First, reconsider the causal argument against the Royals. What is to stop its
imaginary defenders from establishing its cogency by advancing a Melnyk-style
inductive case for (KC-Closure)? Not a shortage of positive instances: everywhere you
look, you will find events with non-Champion Royals causes. Not the existence of
counterinstances: there are none. If there is an objection to be made against such an
inductive case, it must be that (KC-Closure) is not susceptible to inductive confirmation.
But why not? Again, I suggest it is because accepting such an inductive case would
commit one to accepting the counterfactual that (KC-Closure) would be true even if the
Champion Royals existed, and this counterfactual should be rejected given the fact—
20
Explaining Causal Closure
obvious even to those who do not know whether the Royals won the 2003 World
Series—that if (KC-Closure) is true at all, it is true only because the Champion Royals do
not exist.
If you want to resist last section’s objection to the inductive case for (Closure),
you are obligated to show that your line of resistance would not equally support an
inductive case for (KC-Closure). Otherwise, you will be saddled with the absurd view
that nothing is defective about the causal argument against the Royals, since there is no
other apparent charge besides uncogency to make against it. I can see no way to fulfill
this obligation that does not involve (at least implicitly) positing some sort of explanatory
difference between (Closure) and (KC-Closure).
For instance, you could try proposing that (Closure) is a law of nature while (KCClosure) is not. Laws are paradigm examples of inductively confirmable propositions,
and so this would successfully account for why (Closure) is inductively confirmable
while (KC-Closure) is not. By extension, it would account for why the causal argument
for physicalism is cogent while the causal argument against the Royals is not. In
connection, positing a (Closure) law would allow you to satisfy the counterfactual
commitments that I claim go along with embracing an inductive case. Once again, we see
that a (Closure) law suits the purposes of the causal argument for physicalism extremely
well—so well that it might almost be regarded as an implicit assumption of the argument.
The trouble, again, is that it is an assumption no physicalist can grant, on pain of
otherwise positing the explanatory overdetermination of (Closure)’s truth.
Here is another way to put the argument. Following many philosophers, I assume
there is a close connection between explanation and confirmation. I also assume that
21
Explaining Causal Closure
(KC-Closure) is not inductively confirmable. If you hold that (Closure) in contrast is
inductively confirmable, then either you must hold that (Closure) has a different sort of
explanation than (KC-Closure) does—which physicalists cannot accept—or you must
somehow sever the connection between explanation and confirmation, drawing an
inductive distinction between the hypotheses that is not grounded in any explanatory
distinction. Maybe this can be done, but the burden is on you to show how.
8.
The Abductive Case for Physicalism
Second, I have an alternative argument for physicalism to offer. This alternative is
similar enough to the causal argument that it is easy to conflate the two, and indeed I
suspect that much of the reason the causal argument has occupied such a central role in
defending physicalism is because it has been confused for this other, better argument.
The two are importantly different, however.
If dualism were true, what evidence should we expect to find of its truth? Well,
one thing to expect is physical effects that are causally unexplainable in terms of physical
causes. In suggesting we should expect to find this, I take for granted that mental events
are non-redundant causes of physical effects—I regard this as something close to a
Moorean fact. What I am assuming, then, is something like (P2) and (P3) of the causal
argument. When we go out and look, however, we do not find this evidence predicted by
dualism. Admittedly, our knowledge of the world’s causal structure is still fairly limited
even when we restrict our attention to causal chains taking place within the body (the
obvious place to look for impingements by a nonphysical mind). But surely it is
significant that those causal chains we do know of consist entirely of physical effects
with physical causes, without even a single clear exception to this pattern.
22
Explaining Causal Closure
Melnyk takes these causal chains of physical events to warrant (Closure)
inductively and directly—that is, without warranting (P*) first. My counterproposal is
that the same causal chains warrant (P*) abductively and directly. That is, (P*)’s truth
best explains why the various causal chains we know of consist entirely of physical
events. Only at this point, having already inferred (P*), do we then go on to deduce
(Closure). The evidence in question does warrant (Closure), then, I concede, but it does
so only indirectly, by first warranting (P*). It’s like the videotape evidence of the 2003
World Series. The videotape warrants belief in (KC-Closure), but only by first warranting
belief that the Royals did not win.
All of the arguments I have advanced in this paper, including even my objection
to the inductive case for (Closure), are compatible with embracing this sort of abductive
case for physicalism.23 Indeed, the abductive case is tailor-made for my arguments, given
that the whole point of the abductive case is to explain positive instances of (Closure) by
appealing to (P*)’s truth rather than to anything else, like a (Closure) law.
One might be tempted to regard the abductive argument as just a trivial variant on
the causal argument. But, no: their different warrant structures mean that the abductive
argument cannot play the role the causal argument is expected to play. This can be
brought out by considering familiar armchair defenses of dualism, like David Chalmers’
zombie argument or Frank Jackson’s knowledge argument.24 Defenders of the causal
argument for physicalism sometimes concede that these dualist defenses have at least
some prima facie pull, while insisting they ultimately must fail. After all, the line goes,
Sometimes the term “induction” is used broadly, to cover abduction as well as enumerative induction. In
that case I concede that (Closure) is “inductively confirmed” by its positive instances, but insist that its
positive instances confirm it only by first confirming (P*).
24
Chalmers (1996), Jackson (1982).
23
23
Explaining Causal Closure
given the compelling evidence for (Closure), if they were successful these dualist
defenses would force us into an intolerable epiphenomenalism (or
overdeterminationism).25 Recall Papineau: “if there were compelling independent
grounds for holding that conscious properties were non-material, then we would have no
option but to accept epiphenomenalism about consciousness.” Causal considerations thus
serve as a cudgel to beat away the false attractions of dualism.
By the lights of the abductive argument, things stand completely differently. The
warrant it provides for (Closure) depends entirely on the warrant it first provides for (P*),
and so any epistemic defeater of (P*) is thereby an undercutting defeater of (Closure).
Our reason for believing that the physical realm is causally closed is just our warranted
belief that physicalism is true, and so if we were to learn that physicalism is not true after
all, we would be left without any reason at all to believe in causal closure. To mimic
Papineau’s formulation: if there were compelling grounds for holding that conscious
properties were non-material, we would no longer possess grounds for accepting causal
closure, and so there would be no reason to avoid interactionist dualism. There is no
causal cudgel here.
Epiphenomenalism is sometimes regarded as the most empirically respectable
form of dualism.26 By the lights of the abductive argument, however, it is merely ad hoc:
dualism predicts a result that we don’t find, and epiphenomenalism is one desperate way
to try to save the dualist in the face of this embarrassment. Compare: belief in unicorns is
empirically disreputable. You don’t make the belief any more reputable by adding that
See for instance Tye (2009: §2.2), a section entitled “Why Consciousness Cannot be Physical,” which
sets out familiar dualist arguments. It is immediately followed by a section (§2.3) entitled “Why
Consciousness Must be Physical,” which defends a version of the causal argument.
26
Jackson (1982) notably defends epiphenomenalism while Chalmers (1996) at least entertains it.
25
24
Explaining Causal Closure
the unicorns are epiphenomenal and that this is why there are no known violations of (UClosure)—you just delve into ad hocery. If you have warrant for believing in unicorns,
you thereby have warrant for rejecting (U-Closure). Similarly, according to the abductive
argument, the entire epistemic cost of dualism is attached to the initial purchase of
nonphysical events. Pay that price and you get the rejection of causal closure for free.
Of course, no armchair defense of dualism could tell us exactly where (Closure) is
violated. That would take empirical investigation. But such a defense could create a
reasonable expectation that (Closure) must be violated somewhere. For a rough analogy,
suppose God comes to us tomorrow and tells us that he created unicorns, but without
telling us anything of their causal status.27 This by itself would defeat whatever prior
warrant we had for believing (U-Closure).
The abductive argument is not in any interesting sense weaker than the causal
argument. After all, the case against unicorns is extremely strong even though it relies on
no cogent causal argument. What we bring out by reflecting on potential defeaters for
physicalism is just what dualism’s problem is alleged to be. The causal argument says it
is a causal problem, and so merely establishing the existence of nonphysical events would
not by itself solve that problem. In contrast, the abductive argument says the problem is
the absence of evidence for the existence of nonphysical events. Imagine that problem
away by supposing that some philosophical argument proved dualism true, and no causal
(or any other) problem would remain.
The abductive argument has several advantages over the causal argument. It does
not require the sort of explanatory overdetermination of (Closure)’s truth that the causal
27
Presumably God could know of this act of creation even if unicorns were epiphenomenal or causally
redundant.
25
Explaining Causal Closure
argument requires. In addition, the abductive argument uses a pattern of reasoning
ubiquitous in scientific and everyday thought, unlike the causal argument which uses a
pattern of reasoning used almost nowhere outside of combating dualists. Surely, it is
relevant to the case for disbelief in unicorns that we know of no effects requiring unicorn
causes: no unicorn hoofprints in the snow, no unicorn droppings in the forest, no visual
experience of unicorns (at least none that cannot be otherwise causally explained). But
just as surely, it’s not that we take this evidence to support (U-Closure) and then build
from there to conclude eventually that there are no unicorns. Rather, this evidence itself
directly (abductively) supports the conclusion that there are no unicorns, without relying
on (U-Closure) as an intermediate result. Why not think that the case against nonphysical
mental events has pretty much the same form as the case against unicorns?
9.
Conclusion
Let me summarize my arguments. If physicalists are to avoid positing explanatory
overdetermination, they must not hold that the physical realm is causally closed for any
reason other than just that everything is physical. But if this is the explanation of causal
closure, it follows that if there were nonphysical events they would have no trouble at all
violating the causal closure of the physical realm. This undermines the intuitive idea
behind the causal argument, and goes against much of what causal argument defenders
have said while advancing their case. But in addition, it suggests a specific flaw in the
causal argument: it is not cogent. Our warrant for believing that the physical realm is
causally closed depends on our prior warrant for believing physicalism itself, in which
case the causal argument cannot be anyone’s basis for knowing the truth of physicalism.
This is disastrous for the causal argument’s sake, but it is not bad news for physicalism
26
Explaining Causal Closure
itself, for there is an alternative abductive argument that gets the warrant structure right
and (relatedly) does not require physicalists to posit explanatory overdetermination.
As a final thought, I concede that I can see why the causal argument would be
attractive to physicalists. It promises a kind of victory even in defeat. If dualism were
proven true, it would be a truth we would very much regret, according to the argument,
because it would threaten to rob us of our mental causation. For the reasons I have set out
in this paper, I believe this thought should be rejected. Physicalists should learn to be
content with victory in victory. Dualism is not a causally problematic doctrine in any
interesting sense, and so nothing catastrophic for mental causation would follow if it were
true. Still, there is more than sufficient reason to think it is not true.
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