(Aprilis), the fourth month of the year, according to the common computation; but the second according to that of the astronomers. It contains 30 days. The word is derived from aperio, I open, because the earth in this month begins to open her bosom for the production of vegetables.

A PRIORI AND A POSTERIORI, philosophical terms, derived from a distinction of Aristotle between the order of nature and the order of human knowledge, that which is prior (προτερον) in the one, being posterior (τελευταῖον) in the other. With the followers of Aristotle, accordingly, a demonstration a priori is a proof from the cause to the effect, a posteriori, a conclusion from the effect to the cause.

The term a priori came afterwards improperly to be applied to any abstract reasoning from a given notion to the conditions which such notion involves, e.g., to what have been called the ontological and cosmological proofs of the existence of God; the proof from experience (e.g., the argument from design) being called a posteriori.

In modern philosophy, however, since the time of Kant (though the distinction may be traced in Hume), these counter terms are used to distinguish those elements of human knowledge which are native, necessary, and antecedent to experience, from those which are merely the data of experience. Thus, the truths of mathematics, founded on the intuitions of time and space are a priori; the facts of history, founded on experience, are a posteriori.