Although unknown to many Western philosophers today, Augustine's De Trinitate is a strikingly original and highly important philosophical treatise. As a whole, the work is an account of the perplexing Christian doctrine that God is both three and one. But, quite surprisingly, the last half is also a treatise on the philosophy of mind; it is, in fact, the first such treatise on mind in the modern sense of “mind”. How Augustine came to write a theological work on the Divine Trinity which is also a treatise on the human mind is an interesting story in itself.

Augustine begins his work by trying to establish the biblical credentials of the Doctrine of the Trinity. Thus Books 1 through 4 are primarily an exercise in biblical exegesis aimed at showing that this doctrine is indeed to be found in the Bible. The next three books, 5 through 7, develop the metaphysical and epistemological distinctions Augustine thinks he needs to discuss the Divine Trinity. Then comes what is philosophically the most exciting part of the work, the last half. It is in that part, Books 8 through 15, that Augustine develops his remarkably original thoughts on the human mind.

To be sure, there are also other works in which Augustine develops thoughts about the mind. One of the most interesting of these, Book 10 of the Confessions, might easily be overlooked as a source for Augustine's philosophy of mind because the explicit topic for discussion there is memoria, memory.