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Modal scepticism, Yablo-style conceivability, and analogical reasoning

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This paper offers a detailed criticism of different versions of modal scepticism proposed by Van Inwagen and Hawke, and, against these views, attempts to vindicate our reliance on thought experiments in philosophy. More than one different meaning of “modal scepticism” will be distinguished. Focusing mainly on Hawke’s more detailed view I argue that none of these versions of modal scepticism is compelling, since sceptical conclusions depend on an untenable and, perhaps, incoherent modal epistemology. With a detailed account of modal defeaters at hand I argue that Van Inwagen and Hawke’s scepticism is either groundless, or it leads to boundless and unacceptable modal scepticism. Additionally, I show that Hawke’s conception of analogical modal reasoning is problematic. Either his principle of similarity is arbitrary or it begs the question about modal scepticism. In contrast to Hawke’s restricted view of analogical modal reasoning, I present two examples of analogy-based modal justification of philosophically relevant possibility claims. My criticism of modal scepticism also shows that there is no good reason to insist on a sharp distinction between an unproblematic and a presumably dubious kind of modality. The upshot is that in absence of proper defeaters both Yablo-style conceivability and properly applied analogical reasoning are reliable guides to possibility, and also that modal justification comes in degrees. The proposed framework of defeaters of modal justification as well as the analysed examples of analogical modal reasoning trace out interesting new areas for further discussions.

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  1. 1.

    Namely, statements of the form “p is possible”, where p is known to be false, or its truth value is unknown. Usually the references to “non-trivial” are in brackets. Nonetheless, if it is not indicated otherwise, “modal claim” and “possibility claim” will always refer to non-trivial modal claims and non-trivial possibility claims respectively.

  2. 2.

    Basically, the problem is that given all we know about conceivability as a cognitive process, we cannot explain how we could gain knowledge about causally isolated state of affairs via conceivability, whereas we have a viable theory about how we gain perceptual knowledge. See: Roca-Royes (2007, pp. 117–118) for another formulation and a detailed discussion of the argument. This problem is analogous to Benacerraf’s problem in mathematics (Benacerraf 1973).

  3. 3.

    Yablo calls this “the objection from naturalism” (Yablo 1993, pp. 3–4). He does not offer a well-articulated reply to this concern. Yablo indicates that appearances of possible states of affairs are like perceptual appearances: they are prime facie evidence. For Yablo the causal isolation concerning modal truths does not raise more serious a worry than it would in mathematics. Independently from this discussion, Chudnoff (2011) argues for a somewhat similar view, namely, that the special phenomenal character of intellectual seemings (including modal judgements) is prima facie evidence.

  4. 4.

    Yablo also distinguishes propositional and objectual imagination: imagining the fact that there is a tiger behind me, and imagining the tiger itself (cf. Yablo 1993, p. 27). Although these are two distinct types of imagining, they usually come together. As Van Inwagen’s and Hawke’s modal sceptical arguments can be formulated presupposing either type of imagining, the following discussion of modal scepticism leaves out this distinction from consideration.

  5. 5.

    Yablo also distinguishes inconceivability from nonconceivability. A proposition p is nonconceivable, if and only if “for every world I can imagine, I do not take that world to verify p” (Yablo 1993, fn. 60, p. 30).

  6. 6.

    This is what Geirsson calls The Completeness Argument (Geirsson 2005, pp. 285–288).

  7. 7.

    Thanks to an anonymous referee for calling attention to this point.

  8. 8.

    Notice that Yablo addresses some similar, if not the same kind of, sceptical worries. The core idea of these sceptical objections is that the subject needs an independent reason to deny or rule out that p is impossible. It is not sufficient to say that S conceives of p as possible, and S is unaware that p is impossible, since all of these are coherent with the fact that p involves an unnoticed impossibility. However, the objection says, you do not have any further evidence for the possibility of p—independently from that p appears to be conceivable for you—and appealing to conceivability as a guide to possibility would make the justification circular. Yablo distinguishes a couple of slightly different versions of the (so-called) Circularity Objection. Eventually, Yablo concludes that none of them succeed in undermining conceivability-based modal justification (cf. Yablo 1993, pp. 12–19). Also, it seems likely that the Circularity Objection—as Yablo formulates it—would entrain a boundless scepticism about conceivability, and perhaps about all kinds of modal justification. It is worth noting that Yablo’s strategy to overcome this kind of worry is to show that we do not need to rule out all alternate possibilities that might be considered as a defeater of the possibility of p (cf. Yablo 1993, pp. 14–15). I follow a somewhat similar strategy in Sect. 4, although my main objection against modal scepticism relies on my proposed account of defeaters.

  9. 9.

    This follows from his safe-explanation theory (i.e. his account of modal justification) and his reformulation of Van Inwagen’s argument (cf. Hawke 2011, pp. 362–363). Van Inwagen makes this claim explicit (cf. Van Inwagen 1998, pp. 70–71).

  10. 10.

    “Therefore, if Yablo’s general thesis is right, and if \(I\) am right in my assertion that in the present state of knowledge no one is able to imagine a possible world in which there are naturally purple cows, it follows that (...) no one is even prima facie justified in believing that naturally purple cows are possible.” (Van Inwagen 1998, p. 78). Van Inwagen suggests that the same can be applied to any arbitrary remote possibility claim. Also see his own summary: Van Inwagen (1998, pp. 80–81).

  11. 11.

    See also Geirsson’s reconstruction of two additional arguments for modal scepticism (Geirsson 2005, pp. 281–285). Both have a conclusion similar to (MS2), although Geirsson does not distinguish different formulations of modal scepticism.

  12. 12.

    See Van Inwagen’s discussion about the “infallible Standard Atlas” where he rejects the notion of logical possibility (Van Inwagen 1998, pp. 71–72).

  13. 13.

    Of course, Van Inwagen maintains that we do have knowledge about trivial remote possibility claims. There is no doubt that, for example, we know that neutron stars are possible (cf. Van Inwagen 1998, p. 76).

  14. 14.

    Van Inwagen allows that we can have knowledge about some philosophically relevant cases which are close to everyday experience, such as the fake barn-case (cf. Van Inwagen 1998, fn. 3, p. 81). Interestingly, however, he does not claim that we can know these possibility claims by Yablo-style conceivability. I argue in Sect. 5 that Hawke’s modal epistemology in some sense excludes that we can have knowledge or even justified belief concerning basic (everyday) possibility claims by means of conceivability.

  15. 15.

    This is Hawke’s analysis of his “favourite mug-scenario” (cf. Hawke 2011, p. 355).

  16. 16.

    One might also worry that transparent iron is impossible. If so, then this example is misleading. As a matter of fact, it does not seem unlikely that iron as a natural kind has some fundamental physical properties (say, density) which can exclude the possibility of transparent iron in the first place. If transparent iron is impossible, then it is hard to see why we can draw any general epistemic principle for conceivability-based justification on the basis of this example. At least, it is an open question whether this is a really helpful example to motivate modal scepticism.

  17. 17.

    The two counterexamples are as follows: (1) we can conceive of LPs that do not pop and click when played, even if we cannot conceive of technological details that would make this possible; (2) we can conceive of a scenario in which John F. Kennedy has died from natural causes, even though we can hardly imagine all the details that make the statement true in that world (cf. Geirsson 2005, pp. 287–288).

  18. 18.

    See Hawke’s analysis of Geirsson’s objection (Hawke 2011, pp. 354–355).

  19. 19.

    In a recent interview Yablo explicitly asserts again that conceivability is a defeasible guide to possibility. He does not seem to commit to such a strict requirement what Van Inwagen and Hawke propose. See: (accessed: 22.03.2015).

  20. 20.

    Yablo argues that it is not sufficient to imagine scenario in which a computer printing out something that is claimed to be a counterexample to the Goldbach conjecture (GC) in order to justify that not-GC is possible. Given that no one knows whether such a counterexample exists, the only way to make sure that I conceived of a hypothetical scenario in which not-GC holds is to conceive of a real proof of not-GC. However, of course, no one has conceived of the proof of non-GC so far (cf. Yablo 1993, pp. 31–32).

  21. 21.

    I am grateful for this point to an anonymous referee.

  22. 22.

    This argument allows that conceivability—in principle—is prima facie evidence for possibility. It is worth noting that, however, Van Inwagen does not share this assumption (cf. Van Inwagen 1998, p. 78).

  23. 23.

    “(...) when I imagine a world of such and such a type, it appears to me that a world of that type could really have existed. But when I take it to verify \(p\), I take it that if a world like that had existed, then \(p\) would have been the case. So, when I imagine a world which I take to verify \(p\) (...) I have it appear to me that \(p\) is possible.” (Yablo 1993, p. 30).

  24. 24.

    See: Yablo (1993, p. 6; fn. 35, p. 13; p. 32). Geirsson (2005, p. 295) also makes this principle explicit.

  25. 25.

    This is what is usually called as evidential defeat. The concept of epistemic defeasibility can be traced back to Chisholm who draws an analogy between ethical and epistemological defeasibility. See: Chisholm (1966, pp. 48–49).

  26. 26.

    My analysis utilises Yablo’s and Geirsson’s discussion about defeaters (cf. Geirsson 2005, pp. 296–298; Yablo 1993, pp. 33–37). The terminology is borrowed from Yablo who also mentions a third kind of standard defeater, namely, offsetting defeater which is slightly weaker than undermining defeater (cf. Yablo 1993, fn. 67, p. 35).

  27. 27.

    It is also clear that scientists do not have to take seriously any ridiculous hypothesis that might be a defeater of a corroborated scientific theory. Perhaps, some considerations about defeaters in the modal context have relevance to the discussion about non-modal scepticism as well.

  28. 28.

    Notice that Hawke’s modal epistemology is at least coherent with either (ED1) or (ED2). As we saw, Hawke argues that stipulating either P itself or another modal claim Q which is more modally controversial than P would not be sufficient for the justification of P (cf. Hawke 2011, pp. 357–358). This is so because we need independent evidence for the possibility of p. If this principle is applied to defeaters (as a defeater itself can be a possibility claim as well), perhaps we get something close to either (ED1) or (ED2).

  29. 29.

    Of course, it does not exclude the possibility that mind-body dualism may be justified by another, more subtle conceivability-possibility argument. Lycan (2009) argues that the standard objections against dualism are not compelling, and a dualist can reasonably adhere to her position.

  30. 30.

    One might also accept that, for example, the possibility of purple cows is not justified because we are ignorant about the details that make purple cow pigments possible [but: Geirsson (2005, p. 299) argues for the contrary claim, namely, perhaps we are not totally ignorant about those details]. At any rate, it remains unclear what general conclusion can be drawn from this otherwise philosophically uninteresting case. Moreover, appealing to ignorance typically is not the best way to defeat initial evidence. Clearly, we should introduce restrictions on the argument from our ignorance, since the same worry can be raised in virtually all everyday cases. In my view, a better way to think about this issue is to ask: “Is our ignorance of the details that make p possible always sufficient to defeat any kind of conceivability-based justification?” My answer would be: “probably, no”.

  31. 31.

    Of course, presumably, this scenario is non-actual, and, thus, it is a far-out possibility.

  32. 32.

    As a matter of fact, in a recent interview Yablo expressed his doubts about Chalmers’s zombie argument. See: (accessed: 22.03.2015). Geirsson (2014) is devoted to argue that the possibility of philosophical zombies is unjustified. However, of course, Yablo and Geirsson are far from being modal sceptics.

  33. 33.

    See also premise “P*2” of Hawke’s most elaborated version of his argument for modal scepticism (Hawke 2011, p. 363).

  34. 34.

    For example, there are several uncontroversial modal claims that we can know by the Actuality Principle (so, they must be basic), still they are remote from everyday experience (so, they must be non-basic). Consider: “it is possible that Einsteinium’s atomic number is 99”.

  35. 35.

    “(...) it is far from clear what other means we might have for assessing the truth of these [remote] possibility-claims, other than imagination-centered techniques that have been carefully formulated to avoiding the weaknesses of ’mere’ imagining.” (Hawke 2011, pp. 362–363).

  36. 36.

    Be precise, Putnam has two scenarios, the first is about super-spartans, and the second is about so-called super-super-spartans (cf. Putnam 1980, p. 30). Nevertheless, for the sake of simplicity, I refer only to the super-spartans case.

  37. 37.

    We can move even further and argue that possibility itself is a matter of degree. Kment (2014) has developed the view that “possibility” and “necessity” are not absolute terms, they come in degrees. Kment elucidates the notions of possibility and necessity in terms of the closeness to the actual world. One advantage of this account is that it can analyse impossible scenarios as well, because for Kment modality should not be interpreted in terms of possible worlds, but, rather, by a set of rules that defines how remote the hypothetical scenario is from the actual world.


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I am particularly grateful to Sonia-Roca Royes, Duncan Pritchard, Luca Moretti, and Federico Luzzi for their helpful comments, corrections, and insightful objections. I also owe my thanks to anonymous referees who raised some objections to an earlier version of this paper. Last, but not least, I would like to offer my special thanks to Peter Hawke for his reply and clarification of his main points.

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Correspondence to Peter Hartl.

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Hartl, P. Modal scepticism, Yablo-style conceivability, and analogical reasoning. Synthese 193, 269–291 (2016).

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  • Analogical reasoning
  • Defeaters
  • Hawke
  • Modal scepticism
  • Van Inwagen
  • Yablo-style conceivability