chalmers@arizona.edu
*[[This is a commentary on John Perry's book Knowledge, Possibility, and Consciousness. It is destined for a symposium on Perry's book in Philosophy and Phenomenological Research.]]
John Perry's book Knowledge, Possibility, and Consciousness is a lucid and engaging defense of a physicalist view of consciousness against various anti-physicalist arguments. It will come as no surprise to Perry to learn that I think his defense does not succeed. In what follows, I will address Perry's responses to the three main anti-physicalist arguments he discusses: the zombie argument (focusing on insentience), the knowledge argument (focusing on indexicality), and the modal argument (focusing on intensions).
Zombies are unconscious creatures that are microphysically identical to conscious beings such as ourselves. If zombies are metaphysically possible, then materialism is false. The zombie arguments aims to establish that zombies are conceivable, infers that they are metaphysically possible, and concludes that materialism is false.
Perry's response is straightforward. He says (pp. 77-80) that zombies will be possible if and only if epiphenomenalism is true. And he says that he cannot see any reason why one would think that zombies (and zombie worlds) are possible unless one were already an epiphenomenalist. So the zombie argument begs the question, by tacitly assuming epiphenomenalism as a premise.
I think that Perry is clearly wrong here. Whatever the merits of the zombie argument, it does not beg the question. To see this, note that the zombie argument is not based on a single premise - the possibility of zombies - but rather on two premises. The first premise is that zombies are conceivable, roughly in the sense that there is no a priori contradiction in the idea of a zombie. The second premise is that if zombies are conceivable in this sense, then they are possible. (Or they are "primarily possible", in that there is a possible world satisfying the relevant primary intension).
Each of these premises is accepted by many philosophers who clearly reject epiphenomenalism. For example, the first premise is accepted by many "type-B" materialists (those who accept an epistemic gap between the physical and the phenomenal but deny an ontological gap), al of whom deny epiphenomenalism: e.g., Ned Block, Chris Hill, Joe Levine, Brian Loar, and many others. The second premise is accepted by many "type-A" materialists (those who deny an epistemic gap between physical and phenomenal) and interactionists, all of whom deny epiphenomenalism: e.g., Frank Jackson (current incarnation), David Lewis, Richard Swinburne, and others. Of course few or none of these people accept both premises. But their mere existence shows that the premises have support that is independent of a prior acceptance of epiphenomenalism.
To go beyond a roll-call, it can be seen straightforwardly that each premise has substantive support that goes beyond epiphenomenalism. The second premise rests on general considerations linking conceivability and possibility. The first premise rests partly on prima facie conceivability intuitions that many share, and partly on deeper considerations concerning the absence of any conceptual linkage between microphysical concepts (which are structural-functional in nature) and phenomenal concepts (which are not). In both cases, whether or not these premises are correct, their support presupposes nothing about epiphenomenalism.
I suppose that Perry might try to argue that the first premise tacitly builds in epiphenomenalism. Given all the non-epiphenomenalists who accept the premise, however, this would seem unlikely: it would require that they be deeply irrational, or have deeply divided minds. Further, Perry says a number of things elsewhere in the book that seem to come close to embracing the first premise (in the relevant sense of conceivability). For example, although he embraces an identity between phenomenal and physical properties, he denies (pp. 87-88) that consciousness supervenes logically on the microphysical (i.e., he denies a conceptual entailment from microphysical truths to phenomenal truths), which suggests that there will be no a priori contradiction in the notion of a zombie. And even in raising questions about zombies, he points to nothing like an incoherence in the notion; at best he points to a tension with certain a posteriori beliefs about the causal role of our own conscious states.
In any case, it seems clear that to reject the argument, Perry has to go beyond considerations about begging the question, and instead give substantive reasons to reject one of the premises. From Perry's overall position, I would expect him to be more likely to reject the second premise than the first. Like other type-B materialists, he can accept that zombies are conceivable in the relevant sense, but deny the principle linking conceivability (in this sense) with possibility. I will return to this matter in the last section.
One final point: it is worth noting that even the possibility of zombies does not obviously entail epiphenomenalism. To see this, note that an interactionist dualist can accept the possibility of zombies, by accepting the possibility of physically identical worlds in which physical causal gaps (those filled in the actual world by mental processes) go unfilled, or are filled by something other than mental processes. The first possibility would have many unexplained physical events, but there is nothing metaphysically impossible about unexplained physical events. Also: a Russellian "panprotopsychist", who holds that consciousness is constituted by the unknown intrinsic categorical bases of microphysical dispositions, can accept the possibility of zombies by accepting the possibility of worlds in which the microphysical dispositions have a different categorical basis, or none at all. So even if the argument had the one-premise structure that Perry suggests, then as long as it was appropriately neutral on the physical character of the world, it would not obviously beg the question in favor of epiphenomenalism.
The knowledge argument holds that there is phenomenal knowledge (e.g. concerning what it is like to see red) that is not deducible from physical knowledge, and infers that phenomenal facts are non-physical facts. Perry's response, essentially, is to analyze phenomenal knowledge as a sort of indexical knowledge. We know that there is indexical knowledge (e.g. concerning my current location) that is not deducible from complete physical knowledge, but this indexical knowledge does nothing to falsify physicalism. If phenomenal knowledge is indexical knowledge, then we can straightforwardly explain the epistemic gap between the physical and phenomenal domains, without requiring any non-physical ontology.
Perry applies this strategy to the new knowledge that (physically omniscient) Mary gains when she sees red for the first time, roughly as follows. Mary gains new knowledge of the form "red things cause experiences of type P". This knowledge crucially involves a phenomenal concept P - a concept of what it is like to have a certain sort of experience. Perry's strategy (pp. 145-48) is to analyze Mary's phenomenal concept P as a demonstrative concept - which he labels this_i - that functions, like other demonstrative concepts, to pick out whatever sort of experience she is currently attending to. Because of an underlying indexical character, knowledge involving demonstrative concepts cannot usually be inferred from complete objective knowledge. So Mary's epistemic gap is reduced to the familiar epistemic gap for indexicals and demonstratives.
My response is straightforward. I think that there are clear reasons to hold that phenomenal knowledge (of the crucial sort) is not indexical knowledge, and that phenomenal concepts (of the crucial sort) are not demonstrative concepts of this sort; so this analysis of the epistemic gap fails. Mary does gain indexical knowledge involving a demonstrative concept, but this indexical knowledge is not the phenomenal knowledge that is central to the knowledge argument, and this demonstrative concept is not the concept that Mary's central new knowledge involves. I have argued for this based on an independently-grounded analysis of phenomenal concepts elsewhere (Chalmers 2002a), but I will give some reasons here.
It is useful to consider analogies with other demonstrative knowledge of types. Let this_S be a demonstrative concept of certain shapes. Jill might tell Jack that she is about to show him her favorite shape. When she shows him a circle, he might form the thought "Jill's favorite shape is this_S", where this_S refers to circles. This is a clearly demonstrative thought. He might also form the thought "Jill's favorite shape is a circle". This is a clearly nondemonstrative thought: the right hand side is not a demonstrative concept, but what me might call a qualitative concept. Finally, he might form the thought "this_S is a circle". This is a substantive, nontrivial thought, taking the form of an identity involving a demonstrative concept and a qualitative concept. This sort of thought is very common with demonstratives - one conceives the object of a demonstration as the object of a demonstration, and at the same time attributes it substantive qualitative properties, conceived non-demonstratively.
On the face of it, Mary's situation is precisely analogous. She knows that she is about to have an experience of the sort usually caused by red things. Upon having the experience, she might form the thought "The experience usually caused by red things is this_I". But at the same time, she might form the thought "The experience usually caused by red things is R", where R is a qualitative concept of the sort of experience she is now having. This is brought out by the fact that just as Jack might think the substantive thought "this_S is a circle", Mary might think the substantive thought "this_I is R". Like Jack's thought, Mary's thought involves attributing a certain substantive qualitative nature to an object that is identified demonstratively. The concept R - her qualitative concept of the sort of experience in question - is not a demonstrative concept at all, as witnessed by the nontriviality of the identity involving the demonstrative concept this_I and R. And Mary's crucial new knowledge in the thought-experiment, the new knowledge that the knowledge argument turns on, is the substantive qualitative knowledge involving R (e.g. that red things cause R experiences), not the relatively uninteresting indexical knowledge involving this_I.
One might be distracted by the fact that Mary's concept R is a new concept, acquired upon having the experience, while Jack's concept "circle" is an old concept. But this is inessential to Jack's case. We can imagine that Jack has never seen a circle before (if one is worried about plausibility, make it another shape), but that on seeing a circle for the first time, he acquires the qualitative concept of circularity. He will then be in the position to think the qualitative thought "Jill's favorite shape is a circle", and to think the substantive demonstrative-qualitative thought "This_S is a circle. Mary's situation is just the same. Upon seeing red for the first time, she is able to form a qualitative concept of that sort of experience (based on her exposure to it), to think qualitative thoughts involving that concept, and to think demonstrative-qualitative thoughts in which both a demonstrative and a qualitative concept are deployed.
Perry might respond by holding that the cases are deeply disanalogous, and that where Joe has two concepts ("this_S" and "circle"), Mary has only one (this_I). But this seems quite false to the phenomenology of the cases, both of which involve substantive knowledge of the form "this is such-and-such", where "such-and-such" is a qualitative predication based on exposure to a demonstrated entity. We can also note that just as it seems that Joe could have had different thoughts of the form "this_S is a square", if he had been exposed to a different shape, it seems that Mary could have had different thoughts of the form "this_I is G" had she been exposed to a different sort of experience, such as an experience of green. So it seems that the qualitative concept of the experiential type is quite different from the demonstrative concept.
I conclude that Perry gives an adequate analysis of certain subsidiary demonstrative knowledge that Mary gains - knowledge that is analogous to demonstrative knowledge in other domains. But Perry gives no adequate analysis of Mary's substantive new non-indexical, non-demonstrative phenomenal knowledge, and in particular gives no way of reconciling this new phenomenal knowledge with the truth of physicalism.
(In Chalmers (2002a), I distinguish three sorts of phenomenal concepts: pure phenomenal concepts (such as R), demonstrative phenomenal concepts (such as this_I) and relational phenomenal concepts (such as "the sort of experience typically caused by red things"). Perry's discussion seems to acknowledge only two sorts, the demonstrative and the relational; or at least, it seems to assimilate the pure phenomenal and the demonstrative with each other.)
One can also make a direct case against any analysis of phenomenal knowledge as indexical or demonstrative knowledge, as follows. In the indexical case, any epistemic gaps disappear from an objective perspective. Say that I am physically omniscient, but do not know whether I am in the USA or Australia (let's imagine that there are appropriate qualitative twins in both). Then I have a certain indexical ignorance, and discovering that I am in the USA will constitute new knowledge. But if someone else is watching from the third-person point of view and is also physically omniscient, they will have no corresponding ignorance: they will know that A is in Australia and that B is in the US, and that's that. There is no potential knowledge that they lack: from their perspective, they know everything there is to know about my situation. So my ignorance is essentially indexical, and evaporates from the objective viewpoint. The same goes for indexical ignorance concerning what time it is, for demonstrative ignorance concerning what this is, and so on. In all these cases, the ignorance disappears from the objective viewpoint: an objectively omniscient observer can know everything there is for them to know about my situation, and there will be no doubts for them to settle.
Now consider Mary's ignorance. From her black-and-white room, she is ignorant of all sorts of facts: what it will be like for her to see red for the first time, what it is like for others to see red, and so on. Only the first of these looks even apparently indexical, so let us focus on that. In this case, a physically omniscient observer may have precisely analogous ignorance: even given his complete physical knowledge, he may have no idea what it will be like for Mary to see red for the first time. So this ignorance does not evaporate from the objective viewpoint. The same goes even more strongly for knowledge of what it is like for others to see red. For any observer, regardless of their viewpoint, there will be an epistemic gap between complete physical knowledge and this sort of phenomenal knowledge. This suggests very strongly that phenomenal knowledge is not a variety of indexical or demonstrative knowledge at all. Rather, it is a sort of objective knowledge of the world, not essentially tied to any viewpoint.
If this is right, then any analysis of phenomenal concepts as indexical or demonstrative concepts fails, and any attempt to explain Mary's epistemic gap in terms of the epistemic gap for indexical or demonstrative concepts fails. If this is right, then Perry's substantive account of Mary's epistemic gap, in terms of the "reflexive content" of certain beliefs, will also fail. Reflexive content is essentially indexical content (tied to the relation of this token to the world), so that an analysis in these terms applies only to broadly indexical knowledge.
A similar diagnosis can be applied to analyses of phenomenal knowledge by many other materialists who seek to acknowledge Mary's epistemic gap and to explain it away: any analysis of phenomenal concepts as indexical, demonstrative, or recognitional concepts will fall prey to similar objections. This does not show by itself that no analysis of phenomenal knowledge can be given that saves both physicalism and the epistemic gap but I think that it renders the prospects dim.
The third anti-physicalist discussion that Perry discusses is the "modal argument". Of course the zombie argument is itself a sort of modal argument. Presumably Perry intends a more general target here, but I think some of the same issues arise. The chapter is divided into a discussion of "Kripke's argument" and of "Chalmers' argument". His discussion of the latter focuses on my use of the two-dimensional framework in arguing against materialism; so this can be seen as filling in the issues that went unaddressed in his earlier discussion, concerning the relationship between conceivability and possibility. I will focus on this issue here.
Perry's discussion in this chapter is harder to follow than in the other chapters of the book, and it is not easy to clearly identify his responses to the arguments in question. His basic strategy appears to be to agree with his opponents that where there is "apparent contingency" (Kripke) or "conceivability" (me) of some mental-physical dissociation, there is some sort of possibility in the vicinity, but to argue that once the possibility is appropriately understood (in terms of reflexive relations to tokens and the like), it does not threaten materialism. I was left unclear, however, on how this strategy bears on the specific argument I put forward.
To summarize the argument briefly (see Chalmers 2002b for a more detailed treatment): Let us say that S is conceivable when it is not a priori that ~S. Any given statement S can be associated with two intensions: a primary intension, which is a function from centered worlds (worlds plus individuals/times) to truth-values, and a secondary intension which is a function from uncentered worlds to truth-values. When S is necessary, it has a necessary secondary intension (and vice versa); when S is a priori, it has a necessary primary intension (and somewhat controversially, vice versa). Let us say that S is primarily possible when its primary intension is true at some centered world. Let P be the complete microphysical truth about the world. Let Q be a phenomenal truth. Then the anti-materialist argument can be put as follows
(1) P&~Q is conceivable
(2) If S is conceivable, S is primarily possible.
(3) If P&~Q is primarily possible, materialism is false.
--
(4) Materialism is false.
Here, the first premise is simply a statement of the epistemic gap, grounded in the a priori coherence of zombies, or in the a priori coherence of inverted spectra, or in Mary's inability to deduce phenomenal truths from physical truths. The second premise is a core principle of the two-dimensional framework, linking apriority and primary possibility. The third premise is not completely obvious (since materialism requires the secondary possibility of P&~Q), but some relatively straightforward argument involving the nature of physical and phenomenal concepts takes one from the relevant primary possibility to a relevant secondary possibility.
(On the third premise: If a world satisfies the primary intension of P, it is at least structurally identical to our world. So either (i) the world is completely physically identical to our world but different in mental respects, so materialism is false, or (ii) it differs at most in the intrinsic microphysical properties that underlie this structure. Exploiting (ii) leads directly to the Russellian "panprotopsychist" position, which I count for present purposes as a version of nonmaterialism. The argument also needs supplementing to handle the role of indexicals and centering, but this is not hard: one can argue that premise 1 is true even when one supplements P with full indexical "locating" knowledge (see below for more on this), and then a corresponding version of premise (3) still goes through.)
Despite Perry's extensive discussion of the two-dimensional framework, I was left unclear on which premise of this argument he would reject, and why. For reasons discussed earlier, he seems to be committed to a version of premise 1. Many of Perry's fellow type-B materialists reject premise 2 (often allowing that it applies in all standard cases, but holding that the case of consciousness is exceptional or unique). Perhaps Perry would reject this premise, but he does not discuss it explicitly, so it is hard to tell. Perhaps more likely, given Perry's sympathy for the idea that where there is apparent contingency, there is possibility, it is not out of the question that he accepts the second premise and rejects the third, holding that the relevant possibility does not trouble materialism. But again, it is hard to tell from Perry's discussion, as he does not address the argument directly, and he does not argue against the relevant premises.
Insofar as I understand Perry's response to the two-dimensional argument (on pp. 198-200), it focuses on the status of an identity such as "Q_R = B_47, where Q_R is the concept of whatever sort of experience is caused by red things, and B_47 is the concept of brain states of a certain sort. He allows that the identity is "apparently contingent" (conceivable in the relevant sense), allows that there is a primary possibility in the vicinity, but explains the relevant primary possibility in terms of the fact that (i) Q_R might have picked out a different phenomenal property, and (ii) B_47 might have picked out a different sensation. In two-dimensional terms (modulo a few complications in the translation that I pass over here), it seems that Perry is accepting: (i) the primary intension of Q_R picks out a different phenomenal property in some centered worlds, and (ii) the primary intension of B_47 picks out a different sensation in some centered world. In effect, this is to explain the primary possibility in terms of the fact that Q_R and B_47 have distinct primary and secondary intensions. As such, it appears that Perry is here allowing premise (2) above, but denying the equivalent of premise (3): there is no argument from the relevant primary possibility to the falsity of materialism.
There are a few things to say here. First, my own argument here is not formulated in terms of identities, precisely because it is hard to mount an argument from the primary possibility of the negation of a physical-phenomenal identity to the falsity of materialism. (Perry characterizes my argument in terms of identity on pp. 188-89, but this is a mischaracterization.) When we invoke the full claim P&~Q, rather than the mere negation of an identity, the argument from primary possibility to the falsity of materialism is much more straightforward. So Perry is addressing something of a straw figure here.
Second, Perry invocation of the relational phenomenal concept Q_R is very odd in this context. This is a deeply extrinsic characterization of the phenomenal property in question, with a very different primary and secondary intension. For the anti-materialist argument it is much better to invoke something closer to a pure phenomenal concept, to accommodate Kripke's insight that for phenomenal concepts, there is no gap between reference-fixers and reference (or between primary and secondary intensions). Perhaps Perry denies that there are any such phenomenal concepts; but if so, he needs to give substantive arguments against the Kripkean claim. In any case, it is clear that by appealing to the relational concept, Perry is making things easy for himself. (At the very least, I would have expected to see the demonstrative concept this_I.) If there are phenomenal concepts with the same primary and secondary intension, then Perry's strategy here will fail. (In fact, even if phenomenal concepts have different primary and secondary intensions, the anti-materialist argument still goes through, but things are more complicated.)
Third, Perry appeals to the possibility that there are centered worlds in which (the primary intension of) B_47 picks out a different sensation. It is hard to see how this is supposed to go, at least if B_47 gives a full microphysical description of the relevant state. As such, it seems that there is no centered world in which B_47 picks out a different physical property. And presumably Perry holds that there are no centered worlds in which that very physical property is a different sensation (that would deny the necessity of identity). One way out might be to endorse the Russellian claim that physical concepts are topic-neutral, fixing reference to different underlying intrinsic properties depending on the state of the world. But this leads only to the "panprotopsychist" loophole, which I don't think Perry is exploiting. So it is hard to see how this move works for him.
I would have expected Perry to embrace an alternative strategy, suggested by his response to the knowledge argument. If A is "I am in the USA", then P&~A is conceivable and primarily possible, but the the primary possibility simply corresponds to the actual world with a different center. Such a centered world yields no argument against materialism. By analogy, Perry might hold that P&~Q is conceivable because of a tacit indexicality in Q, and is primarily possible, but the primary possibility simply corresponds to the actual world with a different center, again yielding no argument against materialism. Here the center would presumably need to be expanded beyond an individual and a time, but that could be justified if phenomenal concepts were truly analogous indexical concepts, so that phenomenal information is a sort of "locating information". For example, one might supplement the "you are here" and "now is here" arrows at the center with "this is here" arrows pointing to certain states as the referents of phenomenal demonstratives. Then the relevant centered world will simply be a world with different locations for the "this is here" arrows.
I think that this move does not work, for more or less the reasons given above. First, phenomenal concepts are not indexical or demonstrative concepts, for reasons given there. Second, phenomenal information does not disappear from the third-person viewpoint, so it is not locating information (it is epistemically objective information), and so not the sort of thing that can be built into the center of a world. Third, while Mary is in the black and white room, one can supplement her physical information with all the locating information one likes - "you are here", "now is here", and "this is here" with arrows pointing to various of her brain states - and she will still be in complete ignorance of what it is like to see red. So unlike the case of my physical location, the epistemic gap here is not closed by adding any amount of locating information. Still, this is an interesting strategy that is worth exploring, especially in the context of Perry's response to the knowledge argument.
In any case: it is possible that I have misunderstood Perry's intended response to the modal argument here. So I would be interested to see how he responds to the straightforward three-premise argument above.
Perry's responses to the anti-materialist arguments are philosophically deep and sophisticated. Nevertheless, I think that upon close examination, these responses fail in straightforward ways. Given Perry's depth and sophistication, I take this to be indirect evidence that the arguments are sound.
Chalmers, D.J. 2002a. The content and epistemology of phenomenal belief. In (A. Jokic and Q. Smith, eds) Aspects of Consciousness. Oxford University Press. [www.u.arizona.edu/~chalmers/papers/belief.html]
Chalmers, D.J. 2002b. Does conceivability entail possibility? In (T. Gendler and J. Hawthorne, eds) Imagination, Conceivability, and Possibility. Oxford University Press. [www.u.arizona.edu/~chalmers/papers/conceivability.html]
Perry, J. 2001. Knowledge, Possibility, and Consciousness. MIT Press.