APRIORISM. - A philosophical term derived from the expression *a priori*. In its most common modern usage, it designates any method of study and research that proceeds and unfolds on the basis of purely rational principles, independently of experience. Theologically, however, *a priori* has a meaning more immediately connected with the medieval use of the expression (v. APRIORISMO). It is of importance in the question of the demonstration of the existence of God (v.).
It is a common doctrine, founded on divine Revelation and defined by the teaching authority of the Church, that man can arrive at certain knowledge of God through consideration of creatures, in which there is the imprint and reflection of divine perfections. This form of knowledge is evidently *a posteriori*. *A priori* instead are the ways proposed by ontologists (v. ONTOLOGISM) such as Gioberti, by innatists such as Descartes, and by some Scholastics such as St. Anselm.
Ontologism makes God the first idea directly intuited, from which all human cognition unfolds. Descartes claims that the idea of God is innate, clear, and distinct. St. Anselm derived the existence of God from the very concept of God (the simultaneous argument). The Church rejects ontologism because it reduces the beatific vision to a natural fact; Cartesian innatism is not condemned by the Church, but finds its full refutation on the philosophical plane. Among Catholics, the argument of St. Anselm is freely discussed.
Even St. Augustine concedes something to *a priori* when he demonstrates the existence of God from the introspection of the spirit, in which the eternal truths shine forth; but it is now critically established that neither this nor the other assertion of the divine illumination of the human intellect allows us to attribute to St. Augustine a form of ontologism, since he never speaks of a direct intuition of the divine essence by our intellect in its natural conditions. Vedi: AGOSTINO; ANSELMO; DIO; ONTOLOGISMO.