A Priori

A Priori (Latin, “from what comes before”), in philosophy, relating to knowledge that is acquired irrespective of experience, that is, by deductive reasoning alone. A priori knowledge is central to certain branches of epistemology, especially to rationalist theories. René Descartes, for example, posited the reason as a faculty independent of experience, and argued for an innate, or a priori, knowledge of the self in his famous statement Cogito, ergo sum (“I think, therefore I am”), which became the starting point for many of his subsequent investigations.

On the other hand, the existence of a priori knowledge is denied by the empiricists, such as David Hume and John Locke, who argue that only that which comes through experience (that is, a posteriori) can truly be said to be knowable. The existence of a priori knowledge has been a fundamental concern in arguments relating to the existence of God. Many philosophers have argued that to deny a priori knowledge is to make it impossible to prove the existence of God as He is not outwardly perceivable.

The existence of a priori truths is often appealed to in ethics, that is to say, that the fundamental moral ideals can be arrived at by the use of reason.