Apriority, Disputability, and Necessity
This chapter shows how the self-evident and, by extension, a priori propositions in general may plausibly be considered necessary. These propositions are best taken to have, as truthmakers, abstract objects and their interrelations. It is also argued that the a priori may be plausibly taken to extend to certain normative truths and to many propositions that, like some perceptual principles discussed in earlier chapters, belong to philosophy itself. As the case of philosophy well illustrates, when a priori propositions are substantive, there may be widespread rational disagreement on them. This is especially clear if, as argued here, beliefs can be rational even if not sufficiently well-grounded to be justified. This possibility implies that someone may rationally, though unjustifiedly, reject even certain self-evident propositions. How this happens is explained, and the chapter also shows both difficulties in identifying rational disagreements and some prospects for resolving them.