With these objections on the table, I now want to consider three important differences between Davidson’s position and my own.16 As we will see, these differences will ensure that none of the problems raised for Davidson in the preceding section will be problems for me.
The first crucial difference is that even when working within a coarse-grained, Davidsonian conception of events,17 I reject the token identity theory. I believe that mental events are not identical to physical events but rather are realized by them, in a sense I hope to make clearer in this work. Imagine that I’m in pain, and that my pain is realized by firing C-fibers. Imagine next that my firing C-fibers are removed and instantaneously replaced with silicon chips which also realize a pain in me. On my view it’s possible for the pain I’m feeling before the replacement to be token identical to the pain I’m feeling after the replacement, even though the original physical realizer of my pain, the firing C-fibers, has been removed. After the replacement, my pain might go on to cause effects, like my wincing, that the firing C-fibers clearly isn’t causing. And so, if events are individuated by their causes and effects, it follows that the pain realized by the firing C-fibers is not identical to the firing C-fibers.18 Thus, I reject the monism in anomalous monism.19
In the preceding paragraph I assumed a coarse-grained, Davidsonian conception of events. Throughout most of this work, though, I’ll be using the more fine-grained property-exemplification account of events originally defended by Jaegwon Kim.20 On Kim’s view, events are instantiations (or “exemplifications”) of properties by things at times. My entire reason for going with the property-exemplification model is just that I find it more convenient in certain ways than the Davidsonian model is – I don’t have any strong views about what the real and true nature of events is. I assume that problems pertaining specifically to mental causation are neither solved nor created by which account of events one uses, and so I don’t intend for my reliance on the property-exemplification model to do any interesting philosophical work for me.
1.3 The Causal Argument for Physicalism
Perhaps an even deeper divergence from Davidson is that in this work I won’t be committing myself to any very specific view about the nature of causation, and so I’ll be remaining neutral on The Principle of the Nomological Character of Causality. Because of this, I’m unable to help myself to Davidson’s argument for the token identity thesis. Or, more to the point (since I already rejected the token identity thesis), I’m unable to help myself to even a modified version of Davidson’s argument aiming to establish just that mental events are determined by physical events in some physicalistically acceptable way.
I don’t think this is a great loss. For Davidson’s argument is a special version of the so-called causal argument for physicalism, and I think there are independent reasons to reject the causal argument. While different philosophers have developed the argument in subtly different ways,21 for present purposes I’ll be focusing on the following canonical version of it.
(P1): The physical realm is causally closed: any physical event which has a cause at some time thas 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 to physical events.22 For the sake of the objections I want to raise I’m willing to grant that each of the argument’s premises is true and that the argument itself is valid.23 Even if the causal argument is sound, I’ll be claiming, there is still something wrong with it.
1.3.1 WARRANT TRANSMISSION
Consider two different epistemic routes a subject S might take in coming to know that (P1) is true. On the first route, S acquires evidence for the following proposition.
(P*): All events are physical events.
On the basis of this evidence, S comes to know that (P*). Next, S draws out various trivial consequences of (P*) and comes to know them too. These consequences include the propositions that every event which took place on March 30th 1983 was a physical event, that all mental events are physical events (i.e., (C) from the causal argument), and that any physical event which has a cause at some time t has a physical cause at t (i.e., (P1)). This is the first epistemic route to knowing that (P1). On the second epistemic route, S acquires evidence directly supporting (P1) – that is, evidence that supports (P1) without doing so by virtue of first supporting (P*). Through this direct evidence, S comes to know that (P1).
Now, consider an epistemic situation (a possible world) where the causal argument is sound but where only the first route to knowing that (P1) is available to subjects. Within that situation, any suitably rational subject who knows that (P1), and thus is in a position to allow the causal argument to get started, will already know that (C) prior to running through the argument. And so, within that epistemic situation no suitably rational subject who knows that (C) will have acquired her knowledge from the causal argument. To clarify the point, let’s take S to inhabit the epistemic situation in question. If S is to be such that she at least could acquire the knowledge that (C) via the causal argument, she will first need to know that (P1). If S doesn’t antecedently know that (P1), then running through the argument won’t be of any help to her in coming to know that (C). But if S already knows that (P1), the she must also know that (P*) and have inferred (P1) from (P*), since this is the only route to knowing that (P1) in the epistemic situation. But then, since S knows that (P*) and (P*) entails (C), it follows that if S is suitably rational – meaning here just that she would infer that (C) if she knew that (P*) – she will know that (C) even prior to running through the causal argument.
In fact, if S knows that (P1) and is suitably rational, she will know that (C) even if she rejects the causal argument because she (mistakenly) rejects either (P2) or (P3) or both: (P*) entails (C) independently of the truth values of (P2) and (P3). What’s more, the causal argument will not provide S with any further reason to believe that (C) that she does not already have – that is, any reason over and above that which is provided by the available evidence supporting (P*) together with the fact that (P*) entails (C). With respect to (C), then, the causal argument is completely epistemically useless to S.
The problem with the causal argument within the epistemic situation we’ve been considering is that even though it’s sound, it would be impossible for a suitably rational subject to come to know its conclusion on the basis of the argument. This way of putting things is meant to resonate with the recent work on warrant transmission done by Crispin Wright, Martin Davies, and others.24 Using the terminology from that literature, the problem with the causal argument within the epistemic situation we are imagining is that, for a suitably rational subject, the only possible warrant for (P1) will be incapable of transmitting across the argument to (C). Or in other words, the argument is not cogent, in a sense of this term which has been used in discussions of warrant transmission.25
So far I haven’t said anything that a proponent of the causal argument must (or should) reject. Such a philosopher can grant that there are (merely) possible epistemic situations in which the causal argument wouldn’t be cogent. What she must deny is just that our actual epistemic situation is relevantly like this. Reconsider the second epistemic route to knowing that (P1), whereby one acquires evidence that directly supports (P1) without first supporting (P*). In an epistemic situation in which this route to knowledge of (P1) is available, the causal argument might be cogent. The question then is, what is the actual world like? Here in the actual world, does (P1) possess a warrant capable of transmitting across the causal argument?26
To establish cogency, what proponents of the causal argument need is a separate argument, an argument that provides a warrant for (P1) but not by virtue of first providing a warrant for (P*). One initially promising option here is to make an inductive argument for (P1), broadly along the lines suggested in the following passage from Andrew Melnyk.
Nor is it 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.27 While there are other conceivable forms that a direct argument for (P1) might take, an inductive argument of this sort strikes me as the strongest. Part of what I’ll be arguing in the remainder of this section is that this sort of inductive argument for (P1) can’t work. If I can succeed in showing this, then it won’t immediately follow that the causal argument isn’t cogent – again, maybe a different sort of argument for (P1) can be made – but the causal argument’s cogency will have been seriously called into question.
1.3.2 THE CAUSAL ARGUMENT’S GUIDING THOUGHT
Before proceeding with the main thread of the discussion here, let me shift away a bit from the apparatus of cogency and warrant transmission and try to provide a more intuitive gloss on what’s at stake here. Suppose it turns out that the causal argument isn’t cogent. What exactly hangs in the balance?
I take the guiding idea behind the causal argument to be that dualists have a special causal problem: if you’re a dualist, you’re going to have to accept epiphenomenalism or some other deeply problematic view regarding mental causation. Think of the dialectic like this. Occasionally, philosophers find themselves drawn to dualism by certain considerations, like the conceivability of zombies or Mary’s room. These philosophers begin to think seriously about becoming dualists. At this moment, proponents of the causal argument rush in and use dualists’ alleged causal problem to cudgel these philosophers back into physicalism. A representative illustration of this dynamic is provided by the following passage from David Papineau.
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.28 If the causal argument isn’t cogent, though, then regardless of whether or not dualism is in fact true, dualists’ alleged causal problem disappears entirely. For if the causal argument isn’t cogent, then whatever reasons we have to believe (P1) depend completely on the prior reasons we have to believe (P*). But, if one is a dualist, then one must take whatever reasons there are that speak in favor of (P*) to be outweighed by those reasons that speak against it. And so, given dualism, one will have no remaining reasons to believe (P1) – whatever reasons one might have previously had to believe (P1) (i.e., prior to one’s conversion to dualism) will have been completely undercut given the rejection of (P*). If one is a dualist, then, one will have absolutely no reason to shy away from giving a thoroughgoing interaction dualist account of mental causation, in flagrant violation of (P1) – giving such an account won’t require one to reject anything one presently accepts. Metaphorically, the idea here is that if the causal argument isn’t cogent, then the entire epistemic cost of dualism will be attached to the initial purchase of non-physical events. Once this cost has been paid, not accepting (P1) is something that gets thrown in for free.
Consider unicorns. No one thinks that unicorns have a special causal problem; no one tries to argue against their existence by contending that if there were unicorns they would be epiphenomenal (or otherwise causally problematic). And this is because though we believe that there are no unicorn effects (i.e., events caused by unicorns), we take our reasons for this belief to depend entirely on our reasons for first thinking that there are no unicorns. Consider: if, say, God came down and whispered in our ears that unicorns really do exist, we would then take our previous reasons for believing that there are no unicorn effects to be completely undercut. If the causal argument for physicalism isn’t cogent, then non-physical mental events are like unicorns in this respect. Pace Papineau, if we were to learn that non-physical mental events exist, we would then have no remaining reason to believe (P1), and thus there would be absolutely no pressure on us at all to become epiphenomenalists.
None of this is to say that if the causal argument isn’t cogent, dualism is more plausible than we presently think it is. After all, even though there’s no causal argument against unicorns to be had, the view that they exist isn’t plausible. However, it is to say that if the causal argument isn’t cogent, then we physicalists will need to give up on arguing against dualists by contending that they have a special causal problem. Unicorns don’t have a special causal problem, dragons don’t have a special causal problem, witches don’t have a special causal problem, etc. Why think that non-physical mental events would be different in this respect from all these other non-existents?
1.3.3 THE METAPHYSICS BEHIND (P1)
Let’s return now to the prospects of making an inductive argument for (P1). I want to temporarily shift the focus away from epistemological questions concerning how we might know that (P1) is true to the metaphysical question of what about the world might explain its truth. Here are two proposals. First, it might be that (P1) is a natural law. Second, it might be that what explains (P1)’s truth is a mere absence of non-physical events, and thus of potential falsifiers of (P1). That is, it might be that (P*) explains (P1).
In considering the first proposal it’s important to bear in mind the distinction between laws and non-lawlike true generalizations. If (P1) is true, this by itself doesn’t entail that it’s a law. Consider a world w in which there are no non-physical events – that is, where (P*) is true. Since (P*) entails (P1), it then follows that (P1) will be true at w. Still, (P1) might not be a law at w. Given the close connection between laws and counterfactuals, whether or not (P1) is a law at w will be reflected in the truth values of certain counterfactuals evaluated with respect to there. So, for instance, consider the following counterfactual.
(CF): If there were non-physical events, (P1) would still be true.
If (P1) is a law at w then presumably (CF) will be true there, while if (P1) is a mere true generalization at w then presumably (CF) will be false there. The important point is that these are both real options. While the truth of (P*) entails the truth of (P1) at w, it leaves it open whether or not (P1) is a law and thus whether or not (CF) is true.
Now, imagine that God is building a world and that the one thing he wants to guarantee about it is that (P1) is true there. Well, he’s got some options for how to do this. First, he could declare that no non-physical events are to be created – that is, that (P*) is to be true. If he did this, he wouldn’t also need to make (P1) a law. The truth of (P*) would be enough to ensure (P1)’s truth even though it’s not a law. Second, he could declare that (P1) is a law. If he did this, he wouldn’t also need to make sure that no non-physical events are created. The law-status of (P1) would be enough to ensure its truth even if there are non-physical events floating around. Third, God could do both these things. That is, he could both declare that no non-physical events are to be created and also declare that (P1) is a law. A God who did all this to guarantee that (P1) is true, though, would be a God with obsessive-compulsive disorder. Or at least, it would be a God who’s done more than what’s really needed.
What this is meant to help illustrate is that in a world in which it’s both the case that (P*) is true and also that (P1) is a law, the truth of (P1) will be, in a sense, overdetermined. This isn’t a form of causal overdetermination, of course. It’s not that (P1)’s status as a law causes it to be true, for instance. Still, it involves there being two completely independent facts – that of there being no non-physical events, and that of (P1) being a law – each of which is by itself fully sufficient for guaranteeing the truth of (P1). We might think of this as explanatory overdetermination. It seems to me that causal overdetermination, of the sort that figures in (P3) of the causal argument, is really just a special case of this broader category of explanatory overdetermination.
At any rate, I claim that if causal overdetermination is problematic, as proponents of the causal argument insist it is when they defend (P3), then the sort of explanatory overdetermination of the truth of (P1) that we’re presently considering is problematic in exactly the same way. Now, what exactly is the problem with overdetermination (of either sort)? Well, I assume the problem isn’t that overdetermination is metaphysically impossible. There are, I take it, possible worlds where every effect produced by a mental event is causally overdetermined, just as there are possible worlds where everybody who dies is killed by a pair of simultaneous gunshots to the heart. Rather, the problem is that it’s difficult to see what could reasonably convince us that we lived in such a world. If we can give fully sufficient causal explanations for all physical events in terms of other physical events, what reason could there be to posit mental causes of physical events in addition? They seem completely gratuitous.
Similarly, I claim, the problem with holding that the truth of (P1) is explanatorily overdetermined in the way described isn’t that such overdetermination is metaphysically impossible. Rather, it’s that it’s hard to see what could convince us that it actually obtains. To see this, suppose that we already know that (P*) is true. Then what could reasonably lead us to suppose in addition that (P1) is a law? Clearly, this won’t cut it: going out and observing a bunch of causal chains that are in compliance with (P1) while observing no causal chains that are in violation of it. Observed compliance with (P1) puts no pressure on us at all to suppose that (P1) is a law, since we already have in hand a fully sufficient explanation for such compliance – the truth of (P*).
1.3.4 OVERDETERMINATION AND THE INDUCTIVE ARGUMENT FOR (P1) I now want to bring these metaphysical conclusions to bear on the causal argument for physicalism taken in conjunction with the inductive argument for (P1). In short, my complaint is that philosophers making this combination of arguments are committed to the kind of problematic explanatory overdetermination of the truth of (P1) we were just considering. As physicalists, such philosophers are committed to holding that (P*) is true. Since (P*) entails (P1), this gives them one explanation of (P1)’s truth. As proponents of the inductive argument for (P1), such philosophers are committed to holding that (P1) is the kind of generalization that can be confirmed by its instances – that is, to holding that (P1) is a law as opposed to a mere true generalization. This gives them a second explanation for the truth of (P1). But this is one explanation too many.
As further confirmation for my charge here that those proponents of the causal argument who hope to make an inductive argument for (P1) treat (P1) as a law, consider the passage from Papineau quoted above. Papineau claims that if dualism were true, epiphenomenalism would follow. I take it then that Papineau holds that (CF) is true: he holds that even if there were non-physical events, (P1) would still be true. According to Papineau’s view, then, (P1) interacts with counterfactuals in just the way laws do, not the way mere true generalizations do.
Before explicitly connecting the present point back to cogency and warrant transmission, let me press a few separate (though perhaps related) concerns here. First, the view that (P1) is a law seems to be no commitment of physicalism itself. It would seem that a perfectly good physicalist could hold on the one hand that there are no non-physical events and thus that (P1) is in fact true while on the other hand that if there were non-physical events (P1) would be false. By analogy, one can be a unicorn-denier in good standing who holds on the one hand that there are no unicorns but on the other hand that if there were, there would be unicorn effects. Thus, in making an argument that requires (P1) to be a law, causal argument proponents are taking on a position that is a good deal stronger than physicalism itself. Now, it may turn out that this is the only viable way to defend physicalism. If so, though, it would be unfortunate for physicalists; it would be far better if we had an argument that allowed us to be agnostic on whether or not (P1) is a law, since physicalism itself doesn’t force us to take one view or another.
Second, I have claimed that explanatory overdetermination of any sort is problematic. This is at least somewhat controversial, though – after all, a number of philosophers have responded to the causal argument by rejecting (P3) and embracing the pervasive causal overdetermination of the effects of mental events. This poses a challenge to those proponents of the causal argument who hope to make an inductive argument for (P1). They need to come up with grounds for holding that their own particular brand of explanatory overdetermination is unproblematic even while the sort of causal overdetermination embraced by those who reject (P3) is problematic. I think we should be skeptical in advance that this needle can be threaded. Intuitively, what’s problematic about causal overdetermination isn’t the causal part, it’s the overdetermination part.
1.3.5 EVIDENCE FROM THE PHYSICAL SCIENCES
Above, I suggested that the underlying problem with overdetermination is epistemic, not metaphysical: even if overdetermination is metaphysically possible, it’s hard to see what could reasonably convince us that it actually obtains. This, I think, is the real problem facing those causal argument proponents who hope to make an inductive argument for (P1). If, as I’ll now argue, they cannot get a transmittable warrant for (P1) in this way, then this is because of the general epistemic problems facing overdetermination views, I believe.
Now, it’s not implausible to think that differently located causal chains we might observe might bear differentially on the inductive case for (P1). As a rough way of marking the sort of distinction I have in mind, let’s separate between inductive evidence for (P1) that comes from the physical sciences, and inductive evidence for (P1) that comes from the neurosciences.29 I want to begin by focusing on evidence from the physical sciences, the traditional focus of proponents of the causal argument.30 One reason for this traditional focus is the following not implausible thought: if we have any reason at all to think that the entire physical realm is causally closed, that reason will be provided by physics; not by, say, functional neuroanatomy.
To give ourselves a concrete example of the sort evidence from the physical sciences that might support (P1), let’s suppose that we observe a particular chemical process (occurring outside the brain) that terminates in protons being donated from hydrochloric acid molecules mixed with water. Think of this final proton donation event as a physical event having a physical cause at each step in the observed causal chain leading up to it. The question then is, does this piece of evidence, which is to be regarded as but a single representative of a much larger body of similar evidence, give us good (direct) inductive grounds for accepting (P1)?
The answer, I believe, is No. For think about how induction works. As we observe more and more positive instances of a given generalization without observing any counterinstances, we come under more and more pressure to infer that the generalization in question is a law, and thus to infer that there are no counterinstances to it anywhere. That is, we come under pressure to infer lawhood provided that no better explanation of our observations is available. When a better explanation is available our observations often fail to put pressure on us to think that the generalization in question is a law, or perhaps even that it’s true. So, for instance, while we presently know of billions of positive instances and no counterinstances for the generalization that all intelligent life in the universe is on Earth, we don’t regard this knowledge as lending much inductive support for this generalization, and we certainly don’t think it supports the hypothesis that the generalization is a law. This is because an alternative explanation of why we have the knowledge we do is available: our ability to observe other parts of the universe is extremely limited.
Applying this general point about induction to the present case, the observed causal chain leading up to the proton donation event fails to inductively support (P1) because it fails to give us any reason to think that (P1) is a law without counterinstances. And the reason it fails to do this is because an alternative explanation of the causal chain’s observed compliance with (P1) is available. Namely, the observed compliance with (P1) is sufficiently explained by the complete absence of non-physical events anywhere in the vicinity of the causal chain in question. Given this complete absence of non-physical events as at least candidate causes, of course the observed causal chain complies with (P1) – it’s inconceivable that it would fail to do so. It’s not just that there is some alternative explanation or another for the observed compliance that is available here. Rather, this alternative explanation is uncontroversially correct. It’s completely uncontroversial that there are no non-physical events lurking around causal chains taking place outside the brain, like the one leading up to the proton donation event.
I’ve been speaking here of non-physical events failing to “lurk around” or “be in the vicinity of” causal chains like that leading up to the proton donation event. How is this locational talk to be understood? What I mean is that there are no non-physical events that are either (i) spatially proximal to the physical events constituting the observed causal chain, or (ii) nomologically linked to the kinds of events that make up the chain, in a way that would at least potentially call into question whether the observed chain really complies with (P1).31 That there are no non-physical events satisfying either (i) or (ii) is what I take to be uncontroversial.
Maybe there are non-physical events nomologically correlated with physical events in the brain. This is in dispute. What’s not in dispute is that there aren’t any such events nomologically correlated with the types of physical events involved in the observed causal chain. Because of this, trying to gather inductive support for (P1) by observing this causal chain seems a bit like trying to gather inductive support for the hypothesis that fire is epiphenomenal by renting some scuba gear, observing a number of underwater events far away from the nearest fire, and determining that fire hasn’t caused any of these events.
Because we have a fully sufficient, uncontroversially correct explanation of why the observed causal chain complies with (P1), we’re under absolutely no pressure to respond to this observed compliance by inferring that (P1) is a law without counterinstances. What we’re encountering here is really just the sort of evidentiary problem that was noted above in connection with overdetermination. Given the absence of non-physical candidate causes, responding to the observed compliance by inferring that (P1) is a law seems gratuitous. Supposing that (P1) is a law helps us explain absolutely nothing about our observations that we can’t already explain. But, to say that the evidence fails to give us a reason to think that (P1) is a law is just to say that it fails to give (P1) the sort of (direct) inductive support that causal argument proponents are seeking.
The argument I’ve just presented turns in part on the claim that it’s uncontroversial that there are not any non-physical events lurking around the observed causal chain. This feature of the argument might spark concern. After all, the only reason this claim is uncontroversial is because we presently know, thanks to quantum mechanics, that chemical events like those involved in the observed causal chain are physical. If the way we established that chemical events are physical was by reasoning broadly in the style of the causal argument, then it’s illegitimate for me to help myself to this knowledge in the present context. This worry is understandable. It is also unfounded. It’s both the case that the knowledge in question is based on causal argument-style reasoning (or at least very well might be) and also the case that it’s legitimate for me to rely on such knowledge. Let me explain.
For the sake of argument, I’m willing to suppose that the way it was established that chemical events are physical was by first functionalizing chemical event types – that is, by construing them as the types of events that enter into certain characteristic causal relations – and then identifying a specific physical event type (or maybe types) that enters into those causal relations.32 Once we did this, we were licensed to identify physical events of the type in question with chemical events of the type in question. The crucial difference between this procedure and the causal argument as we’ve been considering it is that no inductive leap to (P1) played any role at all here. The vast bulk of the work in carrying out chemical event identifications consisted in the sort of empirical work involved in, say, figuring out the specific physical mechanism responsible for hydrogen bonding. If quantum physicists hadn’t identified these specific physical mechanisms, if they had merely gestured at an inductive case for (P1), we would be far less impressed by their proposed reduction of chemistry to physics.33 If proponents of the causal argument knew of some fairly specific physical mechanisms in the brain that are causally responsible for everything that mental events are supposed to cause, then I would grant in a heartbeat that they had a compelling and cogent argument for (C). They don’t know in any detail what those physical mechanisms are, though. This is why they are forced to make an inductive argument for (P1).
The point I’m making here is closely connected to the thought that while laws are confirmed by their instances, non-lawlike generalizations are not – they are confirmed only through brute enumeration. I am skeptical about whether (P1) is really a law. This skepticism is in no way skepticism about whether it could be shown via brute enumeration that (P1) is true,34 or whether it could be shown via brute enumeration that certain limited causal chains are in compliance with (P1). I take it that a restricted form of this sort of brute enumeration is what takes place when physicists specify the physical mechanisms underlying chemical events. With respect to the distinction between laws and non-lawlike true generalization, there is all the difference in the world between brute enumeration and confirmation by positive instances, and so I am able to help myself to the knowledge that chemical events are physical even as I argue against causal argument proponents that there is no good reason to think that (P1) is a law.
1.3.6 EVIDENCE FROM NEUROSCIENCE
If evidence from the physical sciences fails to give us a good reason to think that (P1) is a law, it would be surprising if evidence from the neurosciences succeeds in doing so. It would be even more surprising if, though (P1)’s status as a law is in question, the neuroscientific evidence showed that some more restricted causal closure principle, applying only to neural events, is a law. In light of these considerations, I think we should be skeptical in advance that neuroscientific evidence can lend the sort of direct inductive support for (P1) that causal argument proponents need.
None of this is to say that our neuroscientific evidence doesn’t provide us with a powerful consideration in favor of physicalism. Surely it does. While our knowledge of the brain’s causal goings on is far from complete at this point, we can at least say that we presently know of no causal chains in the brain clearly in violation of (P1). Surely this bolsters the physicalists’ case. What I deny is just that this bolsters the physicalists’ case in the way that causal argument proponents need. To see how this could be, let me lay out an alternative, absence of evidence argument in favor of physicalism.
Think again of unicorns. The impressive thing about the case against their existence is not that we know of lots and lots of non-unicorns. It would be crazy to try to argue that unicorns don’t exist by going out and observing lots and lots of things that have the property of being a non-unicorn, and then inductively inferring that absolutely everything has this property. Rather, the impressive thing about the case against unicorns is that despite plenty of looking, we don’t know of any. We don’t even know of any evidence that unicorns exist. Given this complete absence of evidence, we have reason to believe that there are no unicorns. As something of an afterthought, given that we have reason to believe this, we also have reason to believe that there are no unicorn effects.
I believe that the case for physicalism based on neuroscientific evidence should be construed broadly along these lines. Given certain assumptions – namely, (P2) and (P3) from the causal argument – we know that a good place to look for evidence for the existence of non-physical mental events is in the form of physical events, presumably located in the brain, lacking fully sufficient physical causes. Though our knowledge of the brain’s causal processes is far from complete at this point, we can at least say that as of right now, we possess no such evidence. Given this complete absence of evidence, we have reason to believe that (P*). As something of an afterthought, given that we have reason to believe that (P*), we also have reason to believe that (P1).
Obviously, this absence of evidence argument for physicalism is extremely similar in certain ways to the causal argument. There’s a crucial difference though. The warrant that the absence of evidence argument supplies for (P1) depends on the warrant it antecedently supplies for (P*). If despite the present absence of evidence, God came down and whispered in our ears that unicorns really do exist – though, we can stipulate, without explicitly telling us anything about their causal status – our prior reasons for holding that there are no unicorn effects would be completely undercut. This is how absence of evidence arguments work: if one eventually acquires some evidence, then whatever conclusions one had reached on the basis of the prior absence of evidence will be completely undercut. Similarly, if despite the present absence of neuroscientific evidence we were to learn that non-physical mental events exist – if, say, David Chalmers came and whispered in our ears that zombies are conceivable and thus possible – then, at least from the standpoint of the absence of evidence argument just set out, we would have no remaining reason to believe that (P1). Thus, the absence of evidence argument for physicalism fails to capture in any way the guiding idea behind the causal argument, which is that dualists have a special causal problem. From the standpoint of the absence of evidence argument, dualists have no more of a causal problem than believers in unicorns do. Instead, their problem is evidentiary – specifically, that there is no evidence to back their view up.
Both the inductive argument for (P1) and the absence of evidence argument say that our present neuroscientific evidence supports (P1). There is a difference though. The former argument says this support is direct while the latter says it is indirect – that is, via (P*). This difference leads to an apparent asymmetry regarding how strong the two arguments need the neuroscientific evidence to be. The inductive argument for (P1) requires the neuroscientific evidence to be extremely strong. It must be strong enough that it would continue to support (P1) even if we were to learn that non-physical mental events exist. It is possible to be a perfectly good physicalist who is thoroughly impressed by the neuroscientific case for physicalism without thinking the evidence for (P1) is quite this robust. It is possible to be a perfectly good physicalist while holding that our neuroscientific knowledge of the brain’s causal chains is presently fairly spotty – spotty enough that if we were to learn that non-physical mental events exist, it would then be unsurprising if some of the causal chains in the brain that we haven’t yet been able to adequately track are in violation of (P1).
The absence of evidence argument will be perfectly agreeable to physicalists of this sort since it doesn’t require nearly so much from our neuroscientific evidence vis-à-vis (P1). The absence of evidence argument requires only that our neuroscientific evidence actually support (P1); it doesn’t require in addition that our evidence continue to support (P1) even if we were to learn that non-physical mental events exist. This is closely connected to a point made above. Proponents of the causal argument who hope to make an inductive argument for (P1) are committed to defending a position stronger than physicalism itself. They are committed to defending physicalism-plus – that is, physicalism plus the further thesis that (P1) is a law. The absence of evidence argument commits one just to physicalism, not physicalism-plus. It would be unsurprising if an argument for physicalism-plus requires more out of our evidence than an argument just for physicalism itself.
Reflection on unicorns prompts the following challenge to those proponents of the causal argument who hope to make an inductive argument for (P1). Can they point to anywhere else in all of science where we reason along the lines suggested by their argument as opposed to the lines suggested by the absence of evidence argument? For just about everything I can think of that we don’t presently believe in – unicorns, dragons, witches, phlogiston, etc. – it seems to me that our reasons for not believing in these things is just that we have no evidence for their existence. It’s not that their existence would be somehow causally problematic. If this is right, if causal arguments are extremely rare and absence of evidence arguments extremely common, it would strongly suggest that when our intuitions tell us that our neuroscientific evidence supports the case for physicalism, our intuitions aren’t tracking the sorts of considerations that causal argument proponents need. Our intuitions aren’t telling us that our neuroscientific evidence shows that (P1) is a law, or, equivalently, that (P1) is directly inductively supported by such evidence. As a physicalist, I find this unsurprising. More specifically, as someone who holds that (P*) is true and who thinks there are genuine epistemic problems facing overdetermination views, what I would find surprising is if there were empirical evidence supporting the hypothesis that (P1) is a law.
If it turns out that the causal argument isn’t cogent, then it would follow that the reason we should hold that the physical realm is causally closed is because we accept physicalism, not vice versa. Intuitively, this strikes me as the proper order of our commitments. Where the causal argument goes wrong is in trying to reverse this order. Bringing this all back to Davidson, I won’t be viewing is as an obligation of the present work to defend claims either about causation in general or about mental causation in particular that somehow feed into an argument for physicalism, ala Davidson in “Mental Events.” Instead, I will take physicalism’s truth more or less for granted and seek to defend a view of the mind/body relation, together with an accompanying account of mental causation, that is compatible with its truth. And so, I’m in general agreement with Jerry Fodor when he writes,
I don’t pretend to do what Davidson seems to think he can, viz., get physicalism just from considerations about the constraints that causation places on covering laws together with the truism that psychological laws aren’t strict. That project was breathtakingly ambitious but maybe not breathtakingly well advised. My guess is, if you want to get a lot of physicalism out, you’re going to have to put a lot of physicalism in.35
1.4 MENTAL CAUSATION
As important as these first two points of divergence with Davidson are, the third one will swamp them throughout the present work: I regard it as a genuine obligation of any account of mental causation to show not just that mental events are causally efficacious, but that they are efficacious thanks to their mental properties. I agree with those critics who charge that Davidson fails to meet this obligation in “Mental Events.” Much of what I will be trying to do in the present work is show that a broadly Davidsonian account of the mind/body relation can be made to square with an account of mental causation that satisfies this obligation.
Now, in “Thinking Causes,” a response to the many critics of “Mental Events,” Davidson makes a move that I want to discuss a bit here. He claims that contrary to what his critics might say, mental properties do matter to causal relations on his account, because they supervene on physical properties. Davidson:
supervenience as I have defined it does, as we have seen, imply that if two events differ in their psychological properties, they differ in their physical properties (which we assume to be causally efficacious). If supervenience holds, psychological properties make a difference to the causal relations of an event, for they matter to the physical properties, and the physical properties matter to causal relations. It does nothing to undermine the argument to say ‘But the mental properties make a difference not as mental but only because they make a difference to the physical properties’. Either they make a difference or they don’t; if supervenience is true, they do.36
There are several problems with this line. First and most seriously, there are good reasons to think that Davidson is simply wrong here and that psychophysical supervenience is perfectly compatible with mental properties being epiphenomenal.37 This point has been widely discussed in the literature, so I won’t dwell on it here. Instead, I want to focus on a problem that arises from the combination of (i) the sort of content externalism originally defended by Putnam and Burge and later embraced by Davidson,38 and (ii) any account of mental causation that tries to ground the causal efficacy of mental properties either in psychophysical supervenience itself or in some other relation – like the realization relation (at least on many views of it) – which entails psychophysical supervenience.
For the sake of the objection I want to raise, I want to grant both that content externalism is true and that wide mental properties can be causally efficacious.39 Now, wide mental properties don’t supervene on intrinsic physical properties, but they do supervene on extrinsic physical properties, perhaps including properties like that of bearing a certain causal relation to H2O (as opposed to XYZ).40 Thus, given Davidson’s acceptance of content externalism, we know that when he appeals to supervenience in the passage just cited, he must mean the supervenience of mental properties on extrinsic physical properties. The thought that the causal efficacy, or at least the “causal-explanatory relevance” (if this is somehow different), of mental properties can be grounded in supervenience (or in some other relation which entails supervenience) is not unique to Davidson. In The Things We Mean, Stephen Schiffer concludes in his discussion of the causal-explanatory role of propositional attitudes that
the most reasonable thing to hold at this stage in the history of our subject is that propositional-attitude facts can’t be identified with physical or topic-neutral facts, but that they do supervene on physical facts and that it is this supervenience that explains the counterfactual value of propositional-attitude because statements. Ava stepped back because she saw that a car was speeding towards her, and this implies that, absent an extremely rare kind of overdetermination, she wouldn’t have stepped back when she did if she hadn’t seen that a car was speeding towards her. This is because if she hadn’t seen that a car was speeding towards her, then she wouldn’t have been in a certain neurophysiological state that was a cause of her stepping back. And this in turn is because her seeing that a car was speeding towards her supervened on a very large physical state, perhaps one stretching back into the past and taking in very complex relations to all sorts of distal things that included the neurophysiological state, and that neurophysiological state is a part of the large subvening state that wouldn’t have obtained if the propositional-attitude state hadn’t obtained.41 To get at what I think the problem is with view like Davidson’s and Schiffer’s, consider the thought experiment of Ectoplasm Earth.
It seems we can distinguish between physicalism as a general thesis – a thesis about everything – and a more restricted form of physicalism – physicalism as a thesis about minds. So, for instance, imagine that central state materialism were the correct mind/body theory, but that much to our amazement, water isn’t actually H2O but rather the spooky non-physical stuff ectoplasm. Then physicalism as a general thesis would be false but, I take it, physicalism about minds would still be true.42 It’s surprisingly difficult to say what the thesis of physicalism about minds amounts to exactly – for reasons we’ll see shortly – so instead of trying to define it I will rely just on the intuitive grasp of the notion I take it we presently have.
Let’s suppose that physicalism as a general thesis is true at the actual world, and thus that physicalism about my mind is true here too. Now consider a single possible world which contains a pair of intrinsic duplicates of me, one who lives on the planet Ectoplasm Earth and another who lives on the planet Twectoplasm Earth. The physical environments of these two planets are exactly alike, and each is a great deal like (actual) earth. The one difference between the planets is this: on Ectoplasm Earth, the watery stuff that fills lakes and rivers is the spooky non-physical stuff ectoplasm, while on Twectoplasm Earth the watery stuff that fills lakes and rivers is the distinct spooky non-physical stuff twectoplasm.
If, as we’re supposing, content externalism is true, then for standard Twin Earthian reasons my two duplicates in this possible world will have thoughts with different wide contents. For instance, one will believe that ectoplasm-water is wet, while the other will instead believe that twectoplasm-water is wet. What is novel about the thought experiment is that not only will my two duplicates be alike in all their intrinsic physical properties, they will also be alike in all their extrinsic physical properties. By assumption, what differentiates my two duplicates is not anything physical but rather the different relations they bear to the non-physical stuffs ectoplasm and twectoplasm. But, if there is a mental difference between my two duplicates without a corresponding difference in even their extrinsic physical properties, it follows that mental properties fail to supervene on even extrinsic physical properties at their world.
This has a few interesting implications. First, it would seem to suggest that the restricted thesis of physicalism cannot be understood in terms of supervenience or any other relation that entails supervenience. By assumption, physicalism is true of my mind here in the actual world. But, it does not seem that the mental lives of my intrinsic duplicates in this thought experiment metaphysically differ from my own in any deep or interesting sense. Because of their causal interactions with ectoplasm and twectoplasm (respectively), perhaps my duplicates can think about these non-physical stuffs in ways I cannot, but this point by itself wouldn’t seem to make their minds non-physical in any deep or interesting sense. Thus, I claim, since physicalism about my mind is by assumption true, physicalism about my duplicates’ minds is true, despite the failure of psychophysical supervenience at their worlds. Thus, physicalism about minds doesn’t require the supervenience of mental properties on even extrinsic physical properties.
Relatedly, I claim that however it is that the metaphysics of mental causation work in the actual world, nothing about the conditions stipulated in the thought experiment give us a good reason to think they will work fundamentally differently in my duplicates world. Maybe swapping ectoplasm or twectoplasm for H2O would lead to a drastic change in how aquatic causation works, but such a swap would not by itself seem to alter anything very important about how mental causation works. If mental causation works in the actual world in much the same way it works in my duplicates’ world, though, then given the failure of psychophysical supervenience there, it follows that psychophysical supervenience or any relation that entails psychophysical supervenience cannot be what accounts for the causal efficacy of mental properties here in the actual world.
In reaching this conclusion, I don’t think I’ve implicitly relied on any (illicit) internalist assumptions. The point here isn’t that wide mental properties must be causally inert, or that they metaphysically uninteresting. The point is rather that if content externalism is true, then the fact that mental properties supervene on extrinsic physical properties here in the actual world seems to be something of an accident, at least with respect to how mental causation works and what it is for minds to be physicalistically acceptable. That mental properties so supervene is partly due to certain facts, like that of the actual watery stuff being H2O rather than ectoplasm or twectoplasm, which, it seems, don’t really matter to the issues at hand. If it’s a kind of accident from the standpoint of mental causation that mental properties supervene on physical properties, then it would be a mistake to rely on such supervenience in trying to account for the causal efficacy of mental properties, as Davidson and Schiffer do.
Let me conclude this section by going beyond anything I have proven with this argument. I believe that the proper account of mental causation, when combined with the set of environmental physical facts (if content externalism is true), will explain why mental properties supervene on extrinsic physical properties. This, on my view, is the proper order of explanation. By trying to explain mental causation in terms of supervenience, I believe that Davidson and Schiffer have reversed the proper explanatory order.