Abstract

The subject of this dissertation is the entitlement to modal beliefs, such as the belief that a proposition is necessarily true, or the belief that a proposition is possibly true. My thesis is that the entitlement to modal beliefs has two dimensions, one active and one passive. In the active dimension, someone is entitled to a modal belief just in case he has conducted the appropriate thought experiments. In the passive dimension, someone is entitled to a modal belief just in case the belief fits his evidence, which is constituted by the conceivability or inconceivability of the proposition in question. Thus, my principal contention is that conducting thought experiments, and taking conceivability as evidence of possibility, is both necessary and sufficient for entitled modal belief. Insofar as these methods are a priori, this implies that our entitlement to our modal beliefs is always partially a priori. ;I begin by clarifying the fundamental concepts of entitlement, modality, and apriority. Then I proceed to state and defend a two-dimensional account of the necessary a posteriori. According to such an account, our entitlement to every necessary a posteriori truth is achieved by an a priori deduction from two or more claims, at least one of which is a priori. The a priori dimension of this entitlement involves the very same methods and sources of entitlement as other modal beliefs---thought experiments and states of conceivability or inconceivability. I go on to defend thought experiments and conceivability against skeptical attacks. Much of the criticism of thought experiments turns on a refusal to countenance the relevant sort of modality. However, I defend this sort of modality in an opening chapter of the dissertation. Much skepticism about the epistemic value of conceivability is founded on an illicit demand for a noncircular demonstration of its reliability. However, if universally accepted, this demand would undercut the evidence of the senses as well. Other objections to conceivability are also considered and found wanting. In conclusion, I argue against conventionalism about modality, and defend a form of rationalism in modal epistemology