65. De vera religione 49.97 (BA 8:166–68); my trans.
Many readers know of Descartes' famous "I think, therefore I am" (cogito ergo sum); note that there is an Augustinian version here. From On True Religion:
(49.97) All that I have said about the light of the mind has been manifested to me by nothing other than this very same light. For by this light I understand that what I have said is true and, in turn, by this, I understand that I understand. . . . I understand that I cannot understand unless I am alive, and I understand with greater certitude that by understanding I come to be more alive, more vivacious. Eternal life, after all, surpasses life within time by its very vivaciousness. Nor do I catch a glimpse of what eternity is except by understanding. Naturally, by gazing into the mind, I separate out all changeability from eternity, and discern within eternity itself no period or space of time, because a period of time consists of movements, whether past or future, of things. In eternity there is nothing past and nothing future, because what is past has ceased to be and what is future has not yet begun to be. Eternity, however, simply is. Neither did it use to be, as if it no longer is, nor will it be, as if it is not yet. That is why Eternity alone was able to say to the human mind in the truest sense: "I am who am" (Ex 3:14), and of Eternity it can be said in the truest sense: "The One who is has sent me" (Ex 3:14).65 the oneness of the trinity
In the closing words of On True Religion, Augustine sets out an early (and rhetorically intricate) formulation of his understanding of the Trinity. From On True Religion:
(55.112) Look: I worship one God: the one Principle of all things, and His Wisdom, by whom every wise soul is wise, and His Gift, by whom everyone blessed is blessed. . . . (55.113) Therefore, let our religion (religio) bind (religet) us to the one all-powerful God, because no created thing comes between our minds, by which we understand Him to be Father, and the Truth, that is, the interior light through whom we understand Him. That is why in Him and with Him we venerate the Truth, who is in no way unlike Him and who is the Form of all beings whom the One has made and who, in turn, strive to return to the One. From this it appears to spirit-filled minds that all things have been made by this Form, who alone fills up what all yearn for. But all things would not have been made by the Father through the Son, nor would they be healed and brought to their endpoint unless God were the highest goodness, the One who has no jealousy for any nature whose goodness comes to
65. De vera religione 49.97 (BA 8:166–68); my trans.