A Priori

A priori knowledge is knowledge justified independently of experience — knowledge that is, in principle, accessible to a rational agent who has not yet observed the world. The concept originates in Kant's distinction between a priori and a posteriori knowledge, where the former derives from pure reason and the latter from empirical observation. The distinction has been challenged repeatedly — by Quine's holism, by the Gettier problem, and by the observation that even mathematical reasoning depends on psychologically contingent cognitive architectures. But it persists because something like it is needed: the difference between knowing that all bachelors are unmarried (a conceptual truth) and knowing that snow is white (an empirical fact) is real, even if the boundary between them is messier than Kant supposed.