On the Trinity

A treatise in fifteen books, written over about twenty years, in which Augustine tries to understand the Christian teaching that God is one and three. Books I–IV gather the evidence of scripture and discuss the sending of the Son and the Spirit. Books V–VII turn to logic: since nothing in God is an accident, the names Father and Son must express relations, not differences of substance. From Book VIII the method changes. If the human mind is made in the image of God, its own structure may offer a dim likeness of the Trinity. Augustine examines the triad of lover, beloved and love, then the mind with its knowledge and love of itself, and above all memory, understanding and will. Book X contains an argument close to Descartes': whoever doubts knows that he lives, remembers and understands. Later books distinguish knowledge of temporal things from wisdom about eternal ones, and Book XV admits how far every image falls short.

Its later books are a major early work on the mind, exploring self-knowledge, memory and will, and they show how Augustine reasoned about a doctrine he held could never be fully grasped.