DOES CONCEIVABILITY ENTAIL METAPHYSICAL POSSIBILITY? Moti Mizrahi and David R. Morrow Abstract In this paper, we argue that 'Weak Modal Rationalism', which is the view that ideal primary positive conceivability entails primary metaphysical possibility, is self-defeating. To this end, we outline two reductio arguments against 'Weak Modal Rationalism'. The first reductio shows that, from supposing that 'Weak Modal Rationalism' is true, it follows that conceivability both is and is not conclusive evidence for possibility. The second reductio shows that, from supposing that 'Weak Modal Rationalism' is true, it follows that it is possible that 'Weak Modal Rationalism' is necessarily false, and hence that 'Weak Modal Rationalism' is false. We then argue that adopting a weaker position according to which conceivability is merely prima facie evidence for possibility provides limited protection from our criticism of conceivability arguments. 1 1. Introduction Conceivability arguments have the following structure: 1. It is conceivable that p.  2. It is metaphysically possible that p. For example, David Chalmers 2 offers a conceivability argument along the following lines: 1. It is conceivable that zombies exist.  2. It is metaphysically possible that zombies exist. 3 Conceivability arguments depend on the assumption that conceivability is, in some sense, a guide to metaphysical possibility. 4 In this paper, we argue against that assumption. 1 An earlier version of this paper was presented at the First Annual Philosophers' Cocoon conference at the University of Tampa. We would like to thank Marcus Arvan, Clayton Littlejohn, Robert William Fischer, Ted Parent, Justin Snedegar, Mark Alfano, and Tuomas Tahko for helpful discussion. Finally, we are grateful to several anonymous reviewers for helpful comments on earlier drafts. 2 David Chalmers, The Conscious Mind (New York: Oxford University Press, 1996), pp. 93-171. 3 David Chalmers, The Character of Consciousness (New York: Oxford University Press, 2010), pp. 141206. 4 Tamar Szabó Gendler and John Hawthorne, 'Introduction', in Conceivability and Possibility (New York: Oxford University Press), pp. 5-6. According to Szabó Gendler and Hawthorne (2002, pp. 4-5), metaphysical possibility is often treated as 'primitive' or glossed simply as 'how things might have been', 'how God might have made things', or 'ways it is possible for things to be'. See also David Chalmers, 'Does Conceivability Entail Possibility?' in T. Szabó Gendler and J. Hawthorne (eds.), Conceivability and Possibility (New York: Oxford University Press, 2002), pp. 145-200, who offers a similar gloss (p. 146), and M. O. Fiocco, 'Conceivability, Imagination, and Modal Knowledge', Philosophy and Phenomenological Research 74 (2007), pp. 364-380. Some proponents of conceivability arguments endorse a strong connection between conceivability and possibility, according to which conceivability, properly understood, entails metaphysical possibility. Chalmers defends a version of this assumption, which he calls 'Weak Modal Rationalism': Ideal primary positive conceivability entails primary possibility. 5 In Chalmers' terminology, p 'is ideally conceivable when [p] is conceivable on ideal rational reflection' (emphasis added); 6 p 'is primarily conceivable when it is conceivable that [p] is actually the case' (emphasis added); 7 p 'is positively conceivable when one can imagine that [p]' (emphasis added); 8 p 'is primarily possible if its primary intension is true in some possible world (i.e., if [p] is true in some world considered as actual)' (emphasis added). 9 Of these conditions, the notion of primary conceivability merits further elaboration. Using a possible-worlds framework, to say that it is conceivable that p 'is actually the case' is to say that we can conceive of a world W such that, if W is the actual world, then p is true at W. Chalmers distinguishes this from 'secondary conceivability'. He says that p is secondarily conceivable if it is conceivable that p might have been the case, given the way things actually are. 10 Some things are primarily conceivable but not secondarily conceivable because of a posteriori necessities: Given that Hesperus is actually Venus and that Phosphorus is actually Venus, it could not have been the case that Hesperus was not Phosphorus. Thus, it is not secondarily conceivable that Hesperus is not Phosphorus. If the actual world were one in which the evening star is distinct from the morning star, then Hesperus would not be Phosphorus. (To say that the evening star is distinct from the morning star is to say that the heavenly body that occupies the part of the sky that Venus occupies during the evening in the actual world is distinct from the heavenly body that occupies the part of the sky that Venus actually occupies in the morning.) Since we can conceive of such a world, it is primarily conceivable that Hesperus is not Phosphorus. 11 Like Chalmers, we will focus here on primary conceivability and primary possibility. Others endorse the weaker claim that there is a merely prima facie connection between conceivability and possibility (hereafter 'Defeasible Modal Rationalism'): Conceivability is prima facie evidence of metaphysical possibility, but it does not entail metaphysical possibility. 5 Chalmers, 'Does Conceivability Entail Possibility?' p. 194. 6 Chalmers, 'Does Conceivability Entail Possibility?' p. 147. 7 Chalmers, 'Does Conceivability Entail Possibility?' p. 157. 8 Chalmers, 'Does Conceivability Entail Possibility?' p. 150. On the relation between conceiving, imagining, supposing, and entertaining, see C. Hill, 'Imaginability, Conceivability, Possibility, and the Mind-Body Problem', Philosophical Studies 87 (1997), pp. 61-85. Our argument doesn't turn on the question concerning the nature of conceiving. 9 Chalmers, 'Does Conceivability Entail Possibility?' p. 164. 10 Chalmers, 'Does Conceivability Entail Possibility?' p. 157. 11 Chalmers, 'Does Conceivability Entail Possibility?' pp. 157-159. To endorse 'Defeasible Modal Rationalism' is to say that p's being (ideally, positively, primarily) conceivable constitutes a merely prima facie reason to believe that p is metaphysically possible. 12 In this paper, we argue that 'Weak Modal Rationalism' is self-defeating, and that 'Defeasible Modal Rationalism' provides a weaker foundation for conceivability arguments than proponents of such arguments seem to assume. Here is how we plan to proceed. In Section 2, we present a reductio that, if sound, shows that, from supposing that 'Weak Modal Rationalism' is true, it follows that conceivability both is and is not conclusive evidence for possibility. In Section 3, we present a second reductio which, if sound, shows that, from supposing that 'Weak Modal Rationalism' is true, it follows that it is possible that 'Weak Modal Rationalism' is necessarily false, and hence that 'Weak Modal Rationalism' is false. These two reductio arguments, then, show that 'Weak Modal Rationalism' is self-defeating. In Section 4, we argue that switching from 'Weak Modal Rationalism' to 'Defeasible Modal Rationalism' provides only limited relief from these reductio arguments. 2. Why it is possible that 'Weak Modal Rationalism' is false For our first reductio argument against 'Weak Modal Rationalism', we make use of two rules of inference beyond those of basic propositional logic. The first is the defining axiom of the system of modal logic called S5: If something is possibly true, then it is necessary that it is possibly true. The intuition behind this axiom is that if there is a possible world in which p is true, then that world's existence is necessary, and so it is necessarily possible that p is true. The second is 'Weak Modal Rationalism' itself, which allows us to infer p's primary possibility from p's being ideally, primarily, positively conceivable. The argument runs as follows: (1) 'Weak Modal Rationalism' is true. [assumption for reductio]  (2) If it is ideally, primarily, and positively conceivable that 'Weak Modal Rationalism' is false, then it is primarily possible that 'Weak Modal Rationalism' is false. [from (1)] (3) It is ideally, primarily, and positively conceivable that 'Weak Modal Rationalism' is false. [premise]  (4) It is primarily possible that 'Weak Modal Rationalism' is false. [from (2), (3) by modus ponens] (5) If it is primarily possible that 'Weak Modal Rationalism' is false, then it is necessarily primarily possible that 'Weak Modal Rationalism' is false. [by S5: ◊p → □◊p]  (6) It is necessarily primarily possible that 'Weak Modal Rationalism' is false. [from (4), (5) by modus ponens] (7) If it is necessarily primarily possible that 'Weak Modal Rationalism' is false, then conceivability is not conclusive evidence for possibility. [premise] 12 Szabó Gendler and Hawthorne, 'Introduction', pp. 5-6. See also Fiocco, 'Conceivability, Imagination, and Modal Knowledge', pp. 364-365.  (8) Conceivability is not conclusive evidence for possibility. [from (6), (7) by modus ponens]  (9) But if 'Weak Modal Rationalism' is true, then conceivability is conclusive evidence for possibility. [from (1)]  (10) Conceivability is conclusive evidence for possibility. [from (1), (9) by modus ponens]  (11) Conceivability is and is not conclusive evidence for possibility. [from (8), (10) by conjunction] The soundness of this reductio rests on the truth of premise (3) and premise (7). We defend these premises in this section. Premise (3), which states that it is ideally, primarily, positively conceivable that 'Weak Modal Rationalism' is false, is basically an existence claim. It asserts the existence of some conceivable world in which 'Weak Modal Rationalism' does not obtain. Vindicating premise (3) therefore requires the description of some such world. We sketch a way to conceive of possible worlds in which 'Weak Modal Rationalism' does not obtain. There may be others, as well. Imagine intelligent beings with systematically distorted modal intuitions, akin to the kinds of oddly specific perceptual deficits exhibited by some stroke patients. Conceive, for instance, of beings that consistently misjudge the truth-value of certain kinds of counterfactuals (e.g., counterfactuals about whether some particular object could have been made of a different substance). These beings' modal intuitions are not totally inverted. In particular, they do not regard actual states of affairs to be impossible. They simply regard certain kinds of metaphysically impossible states of affairs to be possible, even after ideal rational reflection. Furthermore, they all share the same modal deficits, leading to erroneous consensus judgements about a posteriori necessities. In a world inhabited by such beings-a world of which we think we can conceive-'Weak Modal Rationalism' would be false, for the beings would be able to conceive of metaphysically impossible worlds. Against this approach, friends of 'Weak Modal Rationalism' might object that the modally-impaired beings we are imagining are not truly conceiving of impossible states of affairs; they merely believe they are doing so. This objection, however, reduces 'Weak Modal Rationalism' to a tautology, for it implies that one has not successfully conceived of a state of affairs unless that state of affairs is genuinely metaphysically possible. We agree that, on that sense of 'conceivable', conceivability entails metaphysical possibility, but we assume that 'Weak Modal Rationalism' is not supposed to be tautological. As for premise (7), which says that, if it is necessarily primarily possible that 'Weak Modal Rationalism' is false, then conceivability is not conclusive evidence for possibility, we take it that entailment is supposed to be a relation of necessary truth preservation. That is, if p entails q, then whenever p is true, q must be true. In other words, if p entails q, then p is conclusive evidence for q. Accordingly, if the relation between 'it is ideally, primarily, and positively conceivable that p' and 'it is primarily possible that p' is entailment, then whenever the former is true, the latter must be true. In other words, if 'it is ideally, primarily, and positively conceivable that p' entails 'it is primarily possible that p', then 'it is ideally, primarily, and positively conceivable that p' is conclusive evidence for 'it is primarily possible that p'. In light of what we have said in defence of premise (3), however, it is possible for 'it is ideally, primarily, and positively conceivable that p' to be true and for 'it is primarily possible that p' to be false. In fact, it is necessarily possible for 'it is ideally, primarily, and positively conceivable that p' to be true and for 'it is primarily possible that p' to be false. But that means that 'it is ideally, primarily, and positively conceivable that p' is not conclusive evidence for 'it is primarily possible that p'. 3. Why it is possible that 'Weak Modal Rationalism' is necessarily false The first reductio outlined in Section 2 makes use of premise (3), which says that it is ideally, primarily, and positively conceivable that 'Weak Modal Rationalism' is false. We think that there is another way to vindicate premise (3). This way relies on the idea that there could be brute metaphysical laws that preclude certain possible worlds that are neither logically self-contradictory nor in violation of some a posteriori necessary truth. For instance, it might be a brute metaphysical fact that there are no possible worlds in which beings with our physical structure lack phenomenal consciousness, just as it is a brute physical fact that nothing can travel faster than light while moving forward in time. If there were brute metaphysical laws-and we think we can conceive of the existence of such laws-then 'Weak Modal Rationalism' would be false, for we could conceive of some seemingly possible worlds that are not, in fact, possible. Thus, we can conceive of the falsity of 'Weak Modal Rationalism'. To give more substance and plausibility to the possibility of brute metaphysical laws, consider the following. Imagine a possible world containing a perfect, omnipotent deity who actively controls every event in that world. Perfection, as we understand it in conceiving of this deity, involves existing, having every good attribute to the maximal degree consistent with having every other good attribute, and having those attributes necessarily. We think we can conceive of such a deity, and we believe that others-such as some medieval Catholic thinkers-have also thought that they could conceive of such a deity. Because such a deity has its attributes necessarily and controls every event in its world, and because the actions of such a perfect being would follow necessarily from its attributes, everything that happens in its world happens necessarily. Call such a world a 'Spinozistic world' and the kind of deity whose existence would lead to a Spinozistic world a 'Spinozistic deity'. (We are not claiming that this is exactly how Spinoza conceived of God, but only that the idea of such a deity is inspired by Spinoza's views. 13 ) If there is a Spinozistic deity, then there is a brute metaphysical law according to which there are no possible worlds other than the actual and necessary one. That means that, if a Spinozistic deity is primarily possible, then it is possible that 'Weak Modal Rationalism' is necessarily false: If such a deity is primarily possible, then there is a possible world, WS, from which no other possible worlds are accessible. Thus, at WS, 'Weak Modal Rationalism' is false because one could conceive of possible worlds that differ from WS, but contrary to the implications of 'Weak Modal Rationalism', those worlds would not be possible if WS were the actual world. Furthermore, it is provable in S5 that, if it is 13 According to Spinoza, 'Nothing in nature is contingent, but all things are from the necessity of the divine nature determined to exist and to act in a definite way' (Ethics, Part I, Proposition 29). Furthermore, 'Things could not have been produced by God in any other way or in any other order than is the case' (Ethics, Part I, Proposition 33). possible that p is necessarily false, then p is false. Thus, the possibility of a Spinozistic deity entails the falsity of 'Weak Modal Rationalism'. More explicitly: (a) 'Weak Modal Rationalism' is true. [assumption for reductio]  (b) If it is ideally, primarily, and positively conceivable that 'Weak Modal Rationalism' is necessarily false, then it is primarily possible that 'Weak Modal Rationalism' is necessarily false. [from (a)] (c) It is ideally, primarily, and positively conceivable that 'Weak Modal Rationalism' is necessarily false. [premise]  (d) It is primarily possible that 'Weak Modal Rationalism' is necessarily false. [from (b), (c) by modus ponens] (e) If it is primarily possible that 'Weak Modal Rationalism' is necessarily false, then 'Weak Modal Rationalism' is false. [by S5: ◊□p → p is provable from p → □◊p] 14  (f) 'Weak Modal Rationalism' is false. [from (d), (e) by modus ponens]  (g) 'Weak Modal Rationalism' is false and 'Weak Modal Rationalism' is true. [from (f), (a) by conjunction] The soundness of this second reductio rests primarily on the truth of premise (c). Now, either a Spinozistic deity is possible or it is not. If it is possible, then so are brute metaphysical laws, since the deity's existence would entail the brute metaphysical law that says that there are no possible worlds other than the actual and necessary one. In that case, our first and second reductio arguments go through. If a Spinozistic deity is impossible, however, its being impossible also entails the existence of brute metaphysical laws: As best we can tell, a Spinozistic deity is logically possible. Thus, if it is not metaphysically possible, there must be some brute metaphysical law ruling out a Spinozistic deity. Hence, both the existence and the non-existence of a Spinozistic world entail the possibility of brute metaphysical laws, which means that brute metaphysical laws must be possible. In that case, our first reductio goes through. Some might object to our argument by distinguishing between modal claims (e.g., 'it is possible that p' and 'necessarily p', where p is a non-modal claim) and meta-modal claims (e.g., 'it is possible that there are no other possible worlds') and then argue that 'Weak Modal Rationalism' applies only to the former, not the latter. Our modal imaginations, on this view, are like telescopes that allow us to peer into other possible worlds. If we can 'see' some state of affairs in some possible world, then there must be some possible world in which that state of affairs exists. But we cannot, on this view, 'zoom out' our imaginative telescopes to see the entire panoply of possible worlds at once. Thus, the rationale for treating conceivability as a guide to possibility would not apply to claims about the existence or non-existence of certain possible worlds. We do not find such a view of modal intuitions especially attractive, but our description of the Spinozistic deity suggests a more principled response to the objection. In describing our Spinozistic deity, we claimed that we can conceive of a deity whose attributes and actions are necessary. That is, we think we can train our imaginative modal telescopes on such a deity. The existence of such a deity entails certain meta-modal 14 James Garson, Modal Logic for Philosophers (Cambridge: Cambridge University Press), p. 43. claims, but the claim that such a deity could exist is merely modal, not meta-modal. To say that such a deity is possible is simply to assert that there is some possible world that satisfies a particular description. Now, either our ability to conceive of a possible world that contains such a deity entails that such a deity is possible or it does not. In the first case, conceivability can be a guide to meta-modal truth by being a guide to modal truths with meta-modal implications. In that case, our argument about brute metaphysical laws goes through. In the second case, there is some possible world-namely, a world containing a Spinozistic deity-whose conceivability does not entail its possibility, in which case 'Weak Modal Rationalism' is false. Thus, this objection to our argument only works if 'Weak Modal Rationalism' is false. Alternatively, one might object to premise (e) of our argument, which states that if it is primarily possible that 'Weak Modal Rationalism' is necessarily false, then 'Weak Modal Rationalism' is false. Since premise (e) is an application of a result of S5, this amounts to saying that S5 is not applicable here. This would be a surprising claim, but the objection is not without merit. After all, Chalmers defines 'primary possibility' in such a way that it is possible that Hesperus and Phosphorus are distinct heavenly bodies. And in a world in which Hesperus and Phosphorus are distinct, it is necessarily false that Hesperus is Phosphorus. Thus, it seems that it is possible that it is necessarily false that Hesperus is Phosphorus. If that is correct, then the logic underlying premise (e) would entail that Hesperus is not Phosphorus. Since Hesperus is Phosphorus, there must be something wrong with that logic in this context. 15 This objection, however, rests on an equivocation between the proposition expressed by 'Hesperus is Phosphorus' in the actual world and the proposition expressed by 'Hesperus is Phosphorus' in some alternative world in which the two heavenly bodies are distinct. Call the respective heavenly bodies in the actual world 'HesperusA' and 'PhosphorusA'. Call the respective heavenly bodies in the alternative possible world 'HesperusP' and 'PhosphorusP'. The logic behind premise (e) entails that HesperusP is not PhosphorusP, but that is consistent with HesperusA being identical with PhosphorusA. Premise (e) does not involve any equivalent equivocation because unlike 'Hesperus is Phosphorus', 'Weak Modal Rationalism' does not contain any rigid designators. To sum up, then, our reductio arguments show that 'Weak Modal Rationalism' is self-defeating. The first reductio, outlined in Section 2, shows that, from supposing that 'Weak Modal Rationalism' is true, it follows that conceivability both is and is not conclusive evidence for possibility. The second reductio, outlined in Section 3, shows that, from supposing that 'Weak Modal Rationalism' is true, the possibility that 'Weak Modal Rationalism' is necessarily false follows, and hence that 'Weak Modal Rationalism' is false. 4. Retreating to prima facie evidence If sound, the reductio arguments outlined in Section 2 and Section 3 show that 'Weak Modal Rationalism' is self-defeating. In this section, we argue that our reductio arguments also create problems for the weaker position that we called 'Defeasible Modal Rationalism'-i.e., the view that conceivability constitutes prima facie evidence (not conclusive evidence) for possibility. 15 Thanks to an anonymous reviewer for pointing this out. To say that p is prima facie evidence for q is to say that an argument with p as the premise and q as the conclusion is defeasibly cogent. To say that an argument is defeasibly cogent is to say that the relation of support between the premise p and the conclusion q can be defeated by further evidence, but that in the absence of any defeaters, the argument is cogent. Following Pollock 16 we can distinguish between rebutters, or reasons that attack the conclusion; reason-defeating defeaters, or reasons that attack a premise; and undercutters, or reasons that attack the connection between the premises and the conclusion without attacking the truth of either. 17 Accordingly, a rebutting defeater for an argument that q is a reason to believe the negation of q or some other proposition that is incompatible with q. 18 Similarly, a reason-defeating defeater for an argument from p to q is a reason to believe the negation of p or some other proposition that is incompatible with p. An undercutting defeater for an argument from p to q is a reason to believe that p is not sufficient grounds for accepting q. 19 Given that our schematic conceivability argument has only one premise-viz., that p is conceivable-any reason-defeating defeater must deny or be incompatible with p's being conceivable. That is, any reason-defeating defeater will take the form 'p is inconceivable' or will entail that p is inconceivable. Some authors have argued for such defeaters for prominent conceivability arguments by arguing, e.g., that zombies are inconceivable 20 or that pain is inconceivable without the presence of its actual physiological underpinnings. 21 Such defeaters are not relevant, however, to our general complaint against conceivability arguments, for even friends of conceivability arguments would allow that such arguments fail when their sole premise is false. A rebutting defeater, by contrast, must deny or be incompatible with p's being metaphysically possible. We can think of only two kinds of defeaters for the claim that p is primarily possible. The first is that p is logically impossible. This would be the case if p were 'Zombies are not zombies'. The second is that there is some brute metaphysical law that precludes p. This would be the case if p were 'Thomas Dewey was President of the United States' and there were, as a brute fact, no possible world in which Dewey was President. Of these two kinds of defeaters, only the latter is logically compatible with p's being conceivable. The former entails that anyone who thinks she has conceived that p is mistaken. Thus, it also counts as a reason-defeating defeater, making it irrelevant to our general complaint against conceivability arguments, for the reasons mentioned above in discussing reason-defeating defeaters. Thus, if 'Defeasible Modal Rationalism' is true- i.e., if conceivability arguments are defeasibly cogent-then the only possible rebutting defeater of interest here is that some logically contingent metaphysical law precludes all possible worlds in which p is true. The crux of our argument against 'Defeasible Modal Rationalism' is that it subtly requires the belief in brute metaphysical laws. Suppose that there are no brute metaphysical laws. In that case, anything is primarily possible unless it is precluded by 16 John Pollock, Contemporary Theories of Knowledge (Savage, MD: Rowman and Littlefield, 1986). 17 Pollock, Contemporary Theories of Knowledge, pp. 38-39. 18 Pollock, Contemporary Theories of Knowledge, p. 38. 19 Pollock, Contemporary Theories of Knowledge, p. 39. 20 Eric Marcus, 'Why Zombies Are Inconceivable', Australasian Journal of Philosophy 82 (2004), pp. 477490 21 Brendan O'Sullivan and Peter Hanks, 'Conceiving of Pain', Dialogue 47 (2008), pp. 351-376. the laws of logic. That is, if there are no brute metaphysical laws, for every proposition that is not logically self-contradictory, there is at least one possible world at which that proposition is true. The only way that something can fail to be (ideally, positively, primarily) conceivable is if it is logically impossible. Thus, if there are no brute metaphysical laws, then everything that is conceivable is metaphysically possible. In that case, conceivability would entail possibility, rather than constituting merely prima facie evidence for it. 'Defeasible Modal Rationalism' would be false; 'Weak Modal Rationalism' would be true. The only stable position, therefore, for the friend of conceivability arguments is to accept the existence of brute metaphysical laws. If there are brute metaphysical laws, then there are some conceivability arguments for which there are rebutting defeaters-viz., those arguments whose conclusions are precluded by brute metaphysical laws. It is unclear how we could ever have epistemic access to brute metaphysical laws. Thus, it is unclear how we could know whether the conclusion of any particular conceivability argument violates a brute metaphysical law. That, in turn, makes it unclear how we could know whether a particular conceivability argument is truly cogent. Even in an epistemically best case scenario, then, the proponent of 'Defeasible Modal Rationalism' could only say that a particular conceivability argument is probably cogent. This, however, may be too optimistic a claim. It is unclear how we could even know how many logically possible worlds are precluded by brute metaphysical laws. Thus, we cannot even know what share of conceivability arguments suffer from rebutting defeaters. For all we know, it could be that most logically possible worlds are, through brute metaphysical laws, metaphysically impossible. In that case, there would probably exist a rebutting defeater for any given conceivability argument. The best a proponent of 'Defeasible Modal Rationalism' could say would be that a particular conceivability argument could be cogent. To be clear, we are not saying that we should refuse to accept the conclusion of any argument about whose cogency we are not certain. Rather, the point is that even in the best possible epistemic situation, the proponent of 'Defeasible Modal Rationalism' could not be in a position to assert that an argument is defeasibly cogent. Furthermore, even in the best possible epistemic situation, the proponent of 'Defeasible Modal Rationalism' cannot know how likely it is that the argument is cogent. We take no stance here on the plausibility of brute metaphysical laws. Our point is merely that one cannot simultaneously maintain 'Defeasible Modal Rationalism' and deny the existence of brute metaphysical laws, and that if one accepts the existence of brute metaphysical laws, then one must acknowledge that every conceivability argument is hostage to deep uncertainty. 5. Conclusion At this point, we can summarise our argument as a dilemma for friends of conceivability arguments. One horn, which we claim skewers those who endorse 'Weak Modal Rationalism', involves the reductio arguments we presented in Section 2 and Section 3. These reductio arguments, if sound, show that 'Weak Modal Rationalism' is selfdefeating. The other horn of the dilemma, which we claim skewers those who endorse 'Defeasible Modal Rationalism', relies on the idea that this position requires the existence of brute metaphysical laws. If there are brute metaphysical laws, we argued, conceivability arguments are inescapably weaker-perhaps much weaker-than their proponents seem to believe. Undercutting such conceivability arguments, of course, does not disprove their conclusions. Despite everything we have said here, there may well be possible worlds populated by the walking phenomenologically dead. Moti Mizrahi Department of Philosophy St. John's University 8000 Utopia Parkway Queens, NY 11439 USA motimizra@gmail.com David R. Morrow Department of Philosophy University of Alabama at Birmingham 900 13 th Street South Birmingham, AL 35294 USA davidmorrow@uab.edu