38. Trin. 9.2.2 (CCL 50:294); trans. McKenna, FOTC 45:271–72. (continued)
On the Trinity 299
(5.8) In these three, when the mind knows itself and loves itself, a trinity remains: the mind, love, and knowledge. And there is no confusion through any commingling, though each is a substance in itself, and all are found mutually in all, whether each one in each two or each two in each one; in any case, all are in all. For the mind is certainly in itself, since it is called a mind in respect to itself, though in relation to its knowledge it is spoken of as loving, as being loved, or as lovable. And knowledge, though it is referred to a mind that either knows or is known, yet in respect to itself it is also spoken of both as known and as knowing, for the knowledge by which the mind itself knows itself is not unknown to itself. And love, though it is referred to as the mind that loves, whose love it is, yet it is likewise love in respect to itself so that it also exists in itself. For love is also loved, nor can it be loved with anything else except with love, that is, with itself. And so each exists in itself. But they are mutually in each other in such a way that the mind that loves is in the love, and the love is in the knowledge of the one that loves, and knowledge is in the mind that knows. And so each one is in each two because the mind that knows and loves itself is in its own love and knowledge; and the love of the mind that knows and loves itself is in the mind and in the knowledge; and the knowledge of the mind that knows and loves itself is in the mind and in its love, because it loves itself as knowing and knows itself as loving. And for this reason each two are also in each one because the mind that knows and loves itself is in the love with its knowledge, and in the knowledge with its love, since the love itself and knowledge are also together in the mind that loves and knows itself. But we have shown above how all are in all since the mind loves itself as a whole and knows itself as a whole, knows all in love and loves all in knowledge, when these three are perfect in respect to themselves. These three, therefore, are in a marvelous way inseparable from one another, and yet each of them is a substance, and all together are one substance or essence, while the terms themselves express a mutual relationship.39
Augustine's Cogito
The French philosopher and mathematician René Descartes (1596–1650) is famous for his terse proof against radical skepticism: "I think, therefore I am" (cogito ergo sum). Augustine had actually proposed a cogito of his own, playing on the fact that the very doubting of existence proves existence. From On the Trinity, Book 10:
39. Trin. 9.5.8 (CCL 50:300–301); trans. McKenna, FOTC 45:277–78. On this issue of self-knowledge and self-love, see Oliver O'Donovan, The Problem of Self-Love in St. Augustine (New Haven: Yale University Press, 1980; reprint: Eugene, OR: Wipf & Stock, 2006), 60–92.