A priori and a posteriori

In epistemology, A priori (‘from the earlier’) and a posteriori (‘from the later’) are the two categories of knowledge, justification, or argument, determining whether or not a proposition is reliant on outside sensory experience; Roughly speaking, a proposition is known or justified a priori if it is known or justified independently of any experience (beyond the experience necessary to understand the proposition); instead, it is known or justified a posteriori if its knowledge and/or justification depends on empirical evidence. For example, the proposition ‘It is sunny in London today’ can be known (if true) a posteriori, whereas the proposition ‘Either it is sunny or it is not sunny in London today’ can be known a priori.

Fields of knowledge where a priori justification is predominant are, for example, mathematics and formal logic; by contrast, most of the sciences generally involve a posteriori justification. In the history of philosophy, the a priori–a posteriori distinction first appeared in the writings of the 14th century logician Albert of Saxony, where the phrases were used to distinguish between arguments ‘from causes to effects’ (a priori) and ‘from effects to causes’ (a posteriori). As an epistemological distinction it became prominent with Immanuel Kant's Critique of Pure Reason, where its relation to the analytic–synthetic distinction is discussed.

In linguistics and language construction, the two terms describe if linguistic content – primarily vocabulary – is derived from any preexisting languages or not.