Abstract
I argue that metaphysicians of mind have not done justice to the notion of accessibility between possible worlds. Once accessibility is given its due, physicalism must be reformulated and conceivability arguments must be reevaluated. To reach these conclusions, I explore a novel way of assessing the zombie conceivability argument. I accept that zombies are possible and ask whether that possibility is accessible from our world in the sense of ‘accessible’ used in possible world semantics. It turns out that the question whether zombie worlds are accessible from our world is equivalent to the question whether physicalism is true at our world. By assuming that zombie worlds are accessible from our world, proponents of the zombie conceivability argument beg the question against physicalism. In other words, it is a mistake to assume that the metaphysical possibility of zombies entails that physicalism is false at our world. I will then consider what happens if a proponent of the zombie conceivability argument should insist that zombie worlds are accessible from our world. I will argue that the same ingredients used in the zombie conceivability argument—whatever exactly they might be—can be used to construct an argument to the opposite conclusion. At that point, we reach a stalemate between physicalism and property dualism: while the possibility of zombies entails property dualism, the possibility of other creatures entails physicalism. Since these two possibilities are mutually inconsistent, either one of them is not genuine or one of them is inaccessible from the actual world. To resolve this stalemate, we need more than traditional conceivability arguments.
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Notes
I took the present working formulation of physicalism from Chalmers (1996) and Jackson (1998), who offer extended defenses of its adequacy. Formulations of physicalism that employ weaker notions of supervenience suffer from difficulties canvassed in Horgan (1993) and Wilson (1999, 2005). Chalmers and Jackson go on to offer a more precise formulation along the lines of Horgan (1982) and Lewis (1983), but I will postpone discussion of that formulation until later. Keeping with the more generic formulation is consistent with what other writers do (e.g., Stoljar 2001, 2005, 2006). Thus formulated, physicalism is weak enough to encompass both reductive and non-reductive forms of physicalism. For instance, according to some versions of functionalism about consciousness, facts about consciousness supervene on physical facts even though the nature of consciousness is functional not physical. Zombies are also discussed in debates specific to functionalism, where they are sometimes defined as creatures behaviorally or functionally identical to us who lack phenomenal consciousness (Searle 1992; Polger 2000). For present purposes, since functional properties are generally assumed to be realized by physical properties, nothing hinges on the putative distinction between physical and functional properties. This notion of physicalism, either reductive or non-reductive, is intended to be in opposition to property dualism, not scientific pluralism [of the kind defended, for instance, by Mitchell (2003)].
I am working with the standard notion of accessibility that is employed in contemporary modal logic. Here is some textual evidence: (1) Kripke (1963a, b) does not use the expression “H2 is accessible from H1” but rather “H2 is ‘possible relative to’ H1,” which he explains by saying that “every proposition true in H2 is possible in H1” (1963a, b, p. 64 in Linsky 1971). (2) Hintikka uses the term “alternative world” and explains it thus: “whatever is possible must be true in some alternative world and… whatever is necessary must be true in all the alternative worlds… only these genuine alternatives really count” (Hintikka 1963, p. 67 in Loux 1979). (3) Hughes and Cresswell say the accessibility of a world is a matter of what is conceivable. This may seem to help the zombie-phile, until we notice that what is conceivable is not unrestricted: it must be restricted by the way the actual world is. So even under Hughes and Cresswell’s construal, the question of whether zombie worlds are accessible from the actual world reduces to the question of whether physicalism is true. (4) Salmon (1989, cf. also Garson 2003) argues that accessibility is a matter of what is possible at a world. Again, the question of accessibility of zombie worlds reduces to the question of physicalism. (5) Bigelow and Pargetter (1991) say it would be desirable if what is accessible from a world would depend on the intrinsic features of that world (cf. McDermott 1999); once again, the question of accessibility reduces to the question of whether physicalism is true at our world.
For instance, Nathan Salmon has argued that S4 (and hence S5) contains an axiom schema some of whose instances are false (Salmon 1989). The axiom schema is that necessary propositions are necessarily necessary, or “□ϕ⊃□□ϕ”. Salmon’s counterexample involves the composition of material artifacts. According to a widely shared intuition, any particular artifact could have originated from matter that is slightly different from its actual original matter; for instance, a particular statue, s, could have originated from a lump of bronze, m′, slightly different from the lump, m, which it actually originated from. But, the same artifact could not have originated from matter that is too different from its actual original matter; a particular statue could not have originated from a lump of bronze, m′, that is sufficiently different from m while remaining the same statue. If this is accepted, it follows that for some matter m’ sufficiently different from m but sufficiently overlapping with m*, s could have originated from m* but could not have originated from m’; but at the same time, if s had originated from m*, then it would have been possible for s to originate from m′. Hence, although it is necessary that s did not originate from m′, it is not necessarily necessary that s did not originate from m′. This example violates the axiom schema mentioned above. Therefore, according to Salmon, S4 (and S5 with it) is fallacious. Salmon believes that the correct logic for metaphysical necessity is probably T (ibid., p. 4).
If critics of S5 are right, we cannot assume that zombie worlds are accessible from the actual world even if we hold the question of physicalism to be a matter of straightforward metaphysical necessity. Rather, we need to discuss which possible worlds are accessible from the actual world. What is the reason to think that the zombie worlds are accessible? The answer is important, for as we have seen, even if we assume that physicalism is a metaphysical claim of the relevant type, determining whether zombie worlds are accessible is equivalent to determining whether physicalism is true.
Metaphysicians ignore models of S5 that lack a universal accessibility relation at their peril. For example, Chalmers (2010, p. 179), following Yablo (1999, pp. 456–457), discusses whether the statement “it is possible that it is necessary that there is an omniscient being” is consistent with the statement “it is possible that it is not necessary that there is an omniscient being”. Chalmers argues that, in S5, these two statements are mutually contradictory, because in S5 “possibly necessarily P” entails “necessarily P”. But, this argument implicitly assumes that all models of S5 are such that accessibility is universal, whereas there are models of S5 where accessibility is non-universal. If the latter are allowed, as they should be in the present context, the two statements are consistent with one another, provided that the two statements are true at worlds that belong to two mutually inaccessible systems of possible worlds.
Similarly, Wedgwood discusses the relation between the metaphysics of mind and accessibility between possible worlds (2000, p. 410 and Appendix). He argues that Kim’s (1993) argument from strong supervenience to global supervenience is valid in S5 but invalid in any logical system weaker than S5. On that basis, Wedgewood argues that non-reductive physicalism can’t hold if we choose S5 as our logic but can hold if we choose weaker logical systems, and therefore non-reductive physicalists should choose a logic weaker than S5. But, Wedgewood’s argument implicitly assumes that all models of S5 are such that accessibility is universal, whereas there are models of S5 where accessibility is non-universal. If the latter kind of model of S5 is considered, Wedgewood’s refutation of Kim’s (1993) argument applies within S5 too, so non-reductive physicalism may hold even under S5. So, contra Wedgewood, non-reductive physicalists need not reject S5 for the reasons offered by Wedgewood.
Cf. Lewis (1983, p. 362): “Materialism is meant to be a contingent thesis, a merit of our world that not all other worlds share.” Both Terry Horgan and Philip Pettit independently told me they hold phenomenal physicalism to be a contingent truth (personal communication). Similar statements are plentiful in the literature.
Following Horgan, a similar formulation was endorsed by Lewis (1983), and then by Jackson (1993, 1994, 1998) and Chalmers (1996, p. 42). There are subtle differences between the formulations offered by these authors, but they do not matter for present purposes. In the main text I adopted Jackson’s version of Horganic physicalism because it is the simplest.
Jackson (1998) argues that Horganic physicalism captures the content of physicalism as follows. If physicalism fails, than Horganic physicalism fails. So far, so good. Jackson also says that if Horganic physicalism fails, then our world must contain something non-physical that is not duplicated by duplicating the physical facts about it, and hence physicalism fails. This is a non sequitur. For it may be that zombie worlds are possible but inaccessible from our world. Physicalism fails at zombie worlds, but this doesn’t show that physicalism fails at our world.
At this point, Jackson might reply that I am misunderstanding the relevant notion of entailment. If the physical facts entail the phenomenal facts, as physicalism maintains, there cannot be worlds with the same physical facts in which the phenomenal facts do not obtain. It would be like saying that the physical facts do not, after all, entail the phenomenal facts, which, again, would show physicalism to be false. My reply is straightforward: entailments between physical facts and phenomenal facts depend in part on what metaphysical laws obtain at a world—we’ve already established that on independent grounds. Otherwise, worlds with disembodied minds, where such entailments clearly fail, would be ruled out by physicalism as well and physicalism would fail to be a contingent thesis. So, some non-zombie worlds are such that physical facts do not entail phenomenal facts. Zombie worlds might be like that too, i.e., they are among the worlds where physicalism may fail. To disprove a metaphysical thesis like physicalism, we need more than a world where physicalism may fail—we need to show that that world is relevant because it’s accessible from ours in the relevant respect.
Another way to put the point is by saying that whether the entailment holds at all worlds depends on what kind of entailment we are appealing to. Different notions of entailment range over more or less comprehensive sets of possible worlds. If Jackson should insist that we are talking about logical entailment in the narrow sense, ranging over all possible worlds, we would have to retreat to the possibility that zombie worlds are inaccessible from our world [or where some aspect of classical logic doesn’t hold; perhaps they are non-standard worlds (Rescher and Brandom 1979) or impossible worlds (Salmon 1989)]. But, we need not go that far, for we already know there are worlds where the entailment doesn’t hold.
Jackson was almost right. In one sense, physicalism is true if and only if zombie worlds are impossible. That is the sense in which worlds that are inaccessible from a world are impossible. They are impossible relative to that world. But, this doesn’t tell us whether conceivability arguments are sound. One point of this paper is that since the question of whether physicalism is true is equivalent to the question of whether zombie worlds are accessible, the zombie conceivability argument, by assuming accessibility, begs the question. Mutatis mutandis, the same holds for analogous conceivability arguments.
Leuenberger (2008) offers an alternative formulation of physicalism (for independent reasons), but his alternative formulation of physicalism is still in the spirit of Horganic physicalism, so Leuenberger ends up having to argue that zombie conceivability arguments do not refute his formulation of physicalism because zombie worlds without blockers are not conceivable. Chalmers (2010, p. 164) insists that they are.
It may seem that rejecting Horganic physicalism as a good formulation of physicalism on these grounds begs the question against the anti-physicalist. From the present perspective, it’s the other way around. Horganic physicalism begs the question of which zombie worlds are accessible from which worlds in favor of the anti-physicalist. That is, by accepting Horganic physicalism, the physicalist took up a huge burden of proof, which makes the task of the dualist easier by comparison. If we want to redress the balance, it is better to accept the possibility of zombie worlds and ask whether they are accessible from our world. Because of this, Horganic physicalism should be amended.
At any rate, Horganic physicalism was a tentative formulation, which may be superseded by better formulations. If we find out that it was flawed—and being refuted is quite a flaw—this is reason enough to look for a better formulation.
The tension between Horganic physicalism and the zombie conceivability argument was already noted by Neil Tennant (1994). Tennant correctly pointed out that another option open to the Horganic physicalist is to conclude that we are zombies. Tennant took this to be a reductio of the view that Horganic physicalism is a correct formulation of physicalism, and so do I.
This qualification is especially germane in light of the dispute, related to the debate about conceivability arguments, about kinds of possibility and necessity. For if, as some allege (Jackson 1998, Chalmers 1996), there is no important distinction to be drawn between logical and metaphysical necessity, and therefore the metaphysical possibility of a world may be established on purely logical grounds, then it becomes very important to distinguish between those possible worlds that are accessible from a world and those that aren’t. For if physicalism and other metaphysical doctrines are to remain contingent, as many people think they should, the set of possible worlds that are relevant to establishing their truth at a given world must be appropriately restricted.
David Chalmers professes such a view (personal correspondence).
Those considerations are the bread and butter of the current debate about conceivability arguments. The problem is, it’s difficult to reach consensus on any of the relevant notions.
Chalmers asserts that physicalism “is not obviously conceivable in any stronger sense” than prima facie negative conceivability (2010, p. 180) and seems to imply that physicalism is inconceivable but, he doesn’t explicitly assert that physicalism is inconceivable and does not identify any contradiction within physicalism. Chalmers is in the same epistemic situation as a physicalist who asserts that zombies are not obviously conceivable in any stronger sense than prima facie negative conceivability, without being able to identify what contradiction lies within property dualism.
For a more explicit definition and detailed discussion of method publicity and epistemic divergence, see Piccinini (2003).
To rule out zombie worlds, we also need to assume that there are phenomenal properties at our world. If we are zombies, then obviously physicalism does not rule out zombie worlds. I am taking for granted that we are not zombies.
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Frankish (2007) independently published an argument that is similar in spirit to that of Sect. 7 below; similar considerations are also found in Marton (1998) and Sturgeon (2000), pp. 114–116. But these authors follow the mainstream in accepting the step from the metaphysical possibility of zombies to the denial of physicalism (Frankish 2007, p. 653), whereas that step is precisely the main target of the present paper. So the argument in Sect. 7 acquires a different significance within the present framework.
Due to other projects, I haven’t had time to publish on this topic until now, although a version of this paper was posted on my website since about 2008. Given the tangled history of this paper and that this paper has been cited (Brown 2010), I decided to publish it in a form that is close to its original 2006 version. Because of this, I do not discuss the possibility of blockers (Hawthorne 2002) and the ceteris absentibus formulation of physicalism it gave rise to (Leuenberger 2008), even though they raise interesting and subtle issues. Briefly: I would recommend handling blockers in the same way that I recommend handling zombies in the main text—that is, I would recommend that any physicalist who holds that physicalism is a contingent thesis deem the possibility of blockers inaccessible from the actual world. I hope to address some of the remaining issues at greater length in the future.
I am grateful to those who commented on this paper in the distant past; I forgot who they were—sorry! For recent comments, I am grateful to the anonymous referees, Dave Chalmers, Terry Horgan, Robert Kirk, Waldemar Rohloff, Stephen Yablo, and the Washington University in St. Louis Philosophy of Science Research Group. Thanks to Brendan Ritchie, Glenn Carruthers, and Liz Schier for inviting me to submit this paper to Topoi. Thanks to Elliott Risch for editorial assistance. This material is based on work supported in part by a University of Missouri research award.
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Piccinini, G. Access Denied to Zombies. Topoi 36, 81–93 (2017). https://doi.org/10.1007/s11245-015-9323-6
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DOI: https://doi.org/10.1007/s11245-015-9323-6