Boghossian, Paul & Peacocke, Christopher (eds.) - New Essays on the A Priori

An a priori proposition is one which can be known to be true without any justification from the character of the subject's experience. This is a brief, pre-theoretical characterization that needs some refinement; but it captures the core of what many philosophers have meant by the notion. Under this intuitive characterization, propositions which are plausibly a priori include the following: the axioms, inference rules, and theorems of logic; the axioms and theorems of arithmetic, and likewise the axioms and theorems of other parts of mathematics and other sciences of the abstract; the principles of the probability calculus; principles of colour incompatibility and implication; some definitions; and perhaps some truths of philosophy itself.