100. Theodicy and Metaphysics. -- St. Augustine proves the existence of God a posteriori from the contingency of the world, from the order of the universe, from the voice of conscience and from the common consent of mankind. His favourite proof is that derived from an interpretation of the characters of necessity and immutability to be found in our intellectual representations and in those root-verities which serve as criterions for our knowledge and conduct. The objects of those judgments and principles can be necessary and immutable only because they are adumbrations of the necessary and immutable essence of God. Therefore God exists (De Lib. Arbitrio, ii., 12 and 15).

Against the Manichaeans he upholds the primordial oneness of an infinitely good and perfect God; against the Neo-Platonists he defends the theory of creation: and creation must have taken place in time, not ab aeterno. Various Alexandrian doctrines are incorporated into St. Augustine's theodicy, -- with, of course, the modifications demanded by his anti-pantheistic attitude. Such, for example, are the inability of man to comprehend God; the transcendence of God above and beyond all categories; His simplicity, eternity, goodness. From the unity of God St. Augustine does not infer Divine unconsciousness (Plotinus). On the contrary, the Divine knowledge is one of the favourite themes of his philosophy. It is here we meet the theory of Exemplarism (see especially L. 83 qq., q. 46), so intimately associated with the name of St. Augustine.

Before building an arch, the architect must first conceive a plan or model of it. In the same way, before creating the universe, God must have conceived its vast design. He knows all possible essences in their relation to His own Divine essence, of which they are the far off imitations (principales formae quaedam vel rationes rerum in divina intelligentia continentur). There is a Divine idea corresponding to each individual contingent thing, the standard or norma of its reality (singula igitur propriis sunt creata rationibus).

As being the primary source of all contingent reality, the Divine ideas are also the ultimate foundation of the intelligibility of all essences, and therefore it is on them finally that the certitude of human knowledge reposes, since our ideas are in conformity with the things around us: neither in these things around us, nor in ourselves, can we discover any sufficient reason for the immutable and necessary truths we possess concerning them.

This Augustinian Exemplarism makes a vital change in the Platonic theory of the Isolated Ideas (22), -- no matter how much St. Augustine may have claimed to be inspired by the Chief of the Academy. Nor does the Augustinian differ any less from the Neo-Platonic theory, wherein the Ideas are a sort of product or inferior emanation of the One Supreme Essence (85, i, b).