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How to undercut radical skepticism


Radical skepticism relies on the hypothesis that one could be completely cut off from the external world. In this paper, I argue that this hypothesis can be rationally motivated by means of a conceivability argument. Subsequently, I submit that this conceivability argument does not furnish a good reason to believe that one could be completely cut off from the external world. To this end, I show that we cannot adequately conceive scenarios that verify the radical skeptical hypothesis. Attempts to do so fall prey to one or another of three pitfalls: they end up incomplete, reveal a deep contradiction or recreate a non-skeptical hypothesis. I use these results to improve upon Pritchard’s (Epistemological disjunctivism. Oxford University Press, Oxford, 2012; Epistemic angst: radical scepticism and the groundlessness of our believing, Princeton University Press, Princeton, 2016) recent attempt at undercutting radical skepticism.

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  1. 1.

    The terminology is from Pritchard (2012, 2016). I come back to this distinction in Sect. 1.

  2. 2.

    In this respect, my project is closer to that of Kung (2011). Nevertheless, we rely on different conceptions of imagination, which result in different undercutting strategies. I compare my approach with Kung’s in footnotes 15 and 17.

  3. 3.

    For similar distinctions, see Cassam (2007), Schiffer (1996: 330), and Williams (1991). I am using ‘intuition’ as roughly equivalent to a pre-theoretical or commonsense commitment. The reader is invited to rephrase the distinction between the two strategies within her preferred account of intuitions.

  4. 4.

    Pritchard (2012: 131-ff., 2014: 221-ff.) concedes that, by engaging in philosophical reflection on our epistemic practices, the putative skeptical paradox can be shown to rest upon intuitive propositions. Nevertheless, he seems to restrict the use of reflection to motivate the underdetermination and the closure principles and not the skeptical hypotheses themselves. Unfortunately, Pritchard does not defend this restriction.

  5. 5.

    I do not mean to imply that any use of conceivability offers an a priori route to rationally motivate modal claims. My claim is rather that conceivability is the natural strategy to rationally motivate radical skeptical hypotheses without introducing empirical grounds. At this stage of my argument, it is an open question whether CA offers an a priori route to rationally motivate radical skeptical hypotheses. I comment upon two other sources of modal knowledge in footnote 19.

  6. 6.

    Defenders of the metaphysical reading include BonJour (2002), Graham (2007), Levin (2000), Kung (2011), Markie (1986), and Pryor (2000). Beebe (2010) argues that a skeptical hypothesis does not need to be metaphysically possible in order to be epistemically significant. His argument is based on an example from theism. If there is a God, the hypothesis that there is no God is necessarily false. Yet, one might use this hypothesis to formulate a skeptical argument against theism. Although I accept the counterexample, I do not think it undermines the present approach. All I need is the claim that the metaphysical possibility of some skeptical hypotheses is—maybe conjoined with other conditions—sufficient to make them epistemically significant.

  7. 7.

    Neta (2004: 300) writes: “[I]t is logically possible for our perceptual experiences to occur just as they do occur whether we are hallucinating or enjoying veridical perception”. Albritton (2011) rejects this logical approach.

  8. 8.

    This reading also fits the line of argument that motivates the so-called ‘highest common factor’ picture of experience (e.g., McDowell 1995). For reasons of space, I cannot examine the relations between CA and the highest common factor picture. All I can say is that my criticism of CA does not directly establish any claim on the nature of perceptual experience. Instead, it establishes the weaker claim that we lack good reasons to think that radical skeptical hypotheses are possible. See Sect. 6.

  9. 9.

    Albritton (2011: 22) seems to be attracted by some form of modal skepticism: “Some such things, and not others, are no doubt possible, if only one knew it. But which ones? Not ‘which ones are not merely possible but actual?’ Just ‘which ones are so much as possible?’ We don’t know, that’s all. By hypothesis we don’t. And neither do skeptics”.

  10. 10.

    There is also a stronger interpretation of transparency, which also holds that introspection does not reveal non-presentational features of perceptual experiences. I do not need to endorse this stronger interpretation in the present context.

  11. 11.

    “‘To what is the subject attending during acts of imagining?’ gets the answer ‘To those things, whatever they are, that figure in the content of the mental states being recreated” (Currie and Ravenscroft 2002: 42).

  12. 12.

    It is not easy to provide an uncontroversial paraphrase of the adjectives ‘real’, ‘external’ or ‘mind-independent’. For my present purposes, these adjectives can be understood as theoretical placeholders for whatever status the intentional objects of our experiences enjoy when they are proper parts of a non-skeptical scenario.

  13. 13.

    This roughly corresponds to Chalmers’ (2002) concept of negative conception and Yablo’s (1993) concept of undecidability.

  14. 14.

    Jackson (1998) holds that one can deduce all psychological truths—including those concerning the phenomenal character of our experiences—from the conjunction of all physical truths. Unfortunately, this view presupposes the possession of empirical knowledge of physical truths, which is not available in the context of radical skepticism (Sect. 2).

  15. 15.

    Albritton (2011) suggests that the question ‘Is it possible or not?’ only makes sense against a background of things known. Thus, radical skeptical possibilities cannot be raised in the first place (see also Levin 2000). Similarly, Kung (2011) argues that our justification to consider radical skeptical scenarios as metaphysically possible rests upon justified beliefs about the external world. My strategy is different. I claim that we cannot perceptually recreate our current perceptual experiences as the experiences of a BIV and that we cannot fill in the details of radical skeptical scenarios by a priori means. Whereas Albritton reaches this conclusion by examining various hypotheses that strike us as ‘silly’ or ‘nonsense’, my arguments exploit some structural features of experiences that prevent us from adequately conceiving radical skeptical hypotheses. Whereas Kung thinks that non-sensory imagination proceeds by stipulation, I emphasize that belief-like imagination cannot bridge the gap between the first- and the third-person perspectives.

    Neta (2004) uses the (related) explanatory gap in order to criticize abductivist solutions to radical skepticism. On his view, if perceptual experiences are conceived subjectively, there will be an unbridgeable explanatory gap between those experiences and the external world conceived objectively. I did not exploit the gap between the first- and the third-person perspectives in order to criticize an anti-skeptical strategy but to block an a priori rational motivation of radical skeptical hypotheses. In addition, the argument does not rely on the claim that the gap is unbridgeable if one conceives of experiences subjectively but on the weaker principle that it cannot be bridged a priori.

  16. 16.

    The conclusion of this sub-section is broadly consistent with Clarke’s (1972) seminal analysis. Still, it differs from Clarke’s approach in a crucial respect. Clarke’s starting point is a set of meta-philosophical claims on the nature of philosophy as a pure inquiry that seeks to provide a detached understanding of the world. Clarke thinks that, although skeptical questions are genuinely philosophical, the skeptic has not managed to put forward genuinely philosophical-skeptical hypotheses. Her hypotheses are “impounded within the plain”. They are not completely detached. If my analysis is correct, it shows that we can reach a similar conclusion without relying on overarching meta-philosophical claims on the nature of philosophy.

  17. 17.

    Kung (2011: 393) makes this point in a different context. In his classical treatment of Cartesian skepticism, Stroud (1984: 268-71) insists that radical skeptical scenarios can be coherently imagined. Still, he treats imagining that p as synonymous with supposing that p and entertaining that p.

  18. 18.

    This corresponds to Sorensen’s (2006) concept of meta-conceivability.

  19. 19.

    Some philosophers think that we have access to possibility by other means, such as intuition (Bealer 2002) or counterfactual reasoning (Williamson 2007). I think that these views cannot be adapted to provide a better rational motivation of radical skeptical hypotheses. Bealer (2002: 73-ff.) construes intuition as a sui generis propositional attitude of ‘seeming’. Thus, if you have the intuition that p, it seems to you that p. Bealer’s view raises a problem, though. Many of us were undecided as to whether radical skeptical hypotheses were genuinely possible and eventually changed our minds on this issue. It is unclear how Bealer could account for this change of mind without relying on the construction of skeptical scenarios in imagination. Williamson (2007) holds that counterfactual reasoning can provide armchair knowledge of modality. Still, he construes armchair knowledge broadly, as involving empirical knowledge. Thus, his approach is not available to rationally motivate radical skeptical hypotheses (Sect. 2).


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I am indebted to Margherita Arcangeli for instructive conversations on the nature of imagination and to Pierre Saint-Germier, whose PhD thesis on the structure of conceivability arguments enabled me to clarify a number of points. I would also like to thank an anonymous referee for useful comments on an earlier draft of this paper. This work was funded by the Swiss National Science Foundation (Research Grant No. 100012-150265/1).

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Correspondence to Santiago Echeverri.

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Echeverri, S. How to undercut radical skepticism. Philos Stud 174, 1299–1321 (2017).

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  • Radical skepticism
  • Modal epistemology
  • Epistemology of perception
  • Imagination