a posteriori

A priori (‘from the earlier’) and a posteriori (‘from the later’) are Latin phrases used in philosophy and linguistics to distinguish types of knowledge, justification, or argument by their reliance on 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.