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Serious theories and skeptical theories: Why you are probably not a brain in a vat

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Skeptical hypotheses such as the brain-in-a-vat hypothesis provide extremely poor explanations for our sensory experiences. Because these scenarios accommodate virtually any possible set of evidence, the probability of any given set of evidence on the skeptical scenario is near zero; hence, on Bayesian grounds, the scenario is not well supported by the evidence. By contrast, serious theories make reasonably specific predictions about the evidence and are then well supported when these predictions are satisfied.

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  1. For a brain-in-a-vat scenario, see Pollock and Cruz (1999, pp. 2–5). For the deceiving God scenario, see Descartes (1984, p. 14).

  2. How should the skeptic understand the notion of “prediction” here? One interpretation is entailment: the BIV scenario could be described such that it just entails that one has the sort of experiences one in fact has. A more flexible interpretation (which leaves both premises 4 and 5 plausible) would be probabilistic: H predicts E provided that P(E|H) > P(E).

  3. For a sample of approaches, see Putnam (1981), Vogel (1990), Dretske (1981), Klein (1995). In Huemer (2001a, pp. 388–389), I provide grounds for rejecting premise 5.

  4. For a brief, early defense of this approach, see BonJour (1985, pp. 183–185). Note that my present defense of the approach does not indicate an abandonment of the direct realist response to skepticism presented in my earlier work (Huemer 2000, 2001b, pp. 181–191); rather, I believe the unsoundness of the skeptical argument is overdetermined.

  5. Keynes (1921), Carnap (1962), Jaynes (2003), Fumerton (2004), Huemer (2009).

  6. The program assigned a random HSV combination to each pixel, from among 16.7 million possibilities. My computer is unable to display 50 of these per second, but a slide show playing ten images per second shows no detectable patterns. The viewer can tell that the image is changing, but one cannot recognize any specific image when it is repeated.

  7. In Huemer (2009), I argue that when possible, the Principle of Indifference should be applied at a more explanatorily basic level, rather than a more superficial level.

  8. Or perhaps the low a priori probability would merely be a symptom of our having justification for rejecting the proposition, with the considerations that explain why the a priori probability is so low constituting the justification. This distinction is immaterial for present purposes, so I shall hereafter elide the distinction.

  9. I thank Daniel Singer for a version of this objection.

  10. Cf. Vogel’s (2010, pp. 416–417) objection to BonJour.

  11. Ramsey (1931, pp. 160–167) and Gillies (2000, pp. 52–53) reject the notion, while Fumerton (1995, p. 218) and Beebe (2009, pp. 628–632) express doubts.

  12. Huemer (2016).

  13. For another argument for this conclusion, based on probability theory, see Huemer (2009, pp. 26–29).

  14. This premise, the “Principle of Inferential Justification,” has been defended by several epistemologists, including Fumerton (1995, p. 36) and Huemer (2016).

  15. For explanation, see Huemer (2009, pp. 10–13).

  16. Newton (1999).

  17. For a brief explanation of the competing accounts of planetary motion, especially retrograde motion, see Smith (2013).

  18. Hume (1975), section IV; Mackie (1977).


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Correspondence to Michael Huemer.

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Huemer, M. Serious theories and skeptical theories: Why you are probably not a brain in a vat. Philos Stud 173, 1031–1052 (2016).

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