ABSTRACT
The previous chapter addressed the problem of epistemic circularity and the problem of epistemic regress, but solving these problems is not much help when it comes to Cartesian skepticism. This is because the core issue of Cartesian skepticism is not epistemic circularity or epistemic regress, but empirically equivalent alternative hypotheses. The natural world hypothesis h NW and the BIV hypothesis h BIV are empirically equivalent to each other in the sense that for any possible reflective sensory evidence e, 1 their likelihoods P(e | h NW) and P(e | h BIV) are the same. As a result, the ratio of their probabilities remains the same no matter what sensory experience e we have: P ( h N W | e ) P ( h B I V | e ) = P ( e | h N W ) P ( h N W ) / P ( e ) P ( e | h B I V ) P ( h B I V ) / P ( e ) = P ( h N W ) P ( h B I V ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203703779/f711bc10-5dc3-4684-a07f-d7ce3405b13a/content/math3.tif"/> It appears as though the dispute over Cartesian skepticism comes down to the comparison of the a priori probabilities, i.e. what probabilities we assign to the two hypotheses h NW and h BIV prior to obtaining any sensory experience. If so, it would be helpful to have some principles that guide the assignment of a priori probabilities. This is a difficult task, and some Bayesians think that a priori probabilities are simply subjective. 2 If that is the case, our choice between the natural world hypothesis and the BIV hypothesis may be a matter of subjective judgment. That might be disappointing to some epistemologists, but perhaps not a disaster because most people’s subjective judgment is in favor of the natural word hypothesis.
