67. II. Demonstrations a Priori, a Posteriori. -- This distinction. which, with modern logicians, takes the place of the preceding, is less rigorous, but has a foundation in the nature of things.

A demonstration is a priori or a posteriori accordingly as the middle term is in reality anterior or posterior to the predicate of the conclusion; it proceeds from the cause or the reason (a causa vel ratione quae in se est prior -- a priori) to the effect or result (ad effectum vel rationatum), or vice versa. -- E. g.: An immaterial subject is imperishable. The human soul is immaterial. Therefore it is imperishable. -- A being which is subject to change requires a cause other than itself to bring about its existence. The universe is subject to change. Therefore there is a cause of the existence of the universe, God.

To this division some authors have added a third member, the demonstration a simultaneo or quasi a priori. It has its place where two things have to be demonstrated which in reality are not distinct, but one of which is necessarily conceived as coming before the other. Such is the proof by which St. Anselm thought it possible to demonstrate the existence of God from the idea of the most perfect Being.