St Augustine here, as his manner is, says things which remain difficult after all explanations given. He finds some analogy to the Blessed Trinity in the human soul thus: "The mind itself and its love and knowledge of itself are three things; and these three are one; and when they are perfect, they are equal." This thought he pursues, De Trinitate IX, Chapp. iii, iv: X, Chapp. iii-x. It makes to his purpose to insist on the equality of the soul's knowledge of itself to the soul as known. "When it knows its whole self, and nothing else with itself, then its knowledge is equal to itself" (De Trin. IX, iv). He frequently repeats that the soul knows its whole self. See especially X, iv, 6. St Thomas would explain: The soul knows its whole self in existence (quod est), but not its whole self in essence (quid est): which is true, but what St Augustine meant is not so clear. Cf. De Trin. X, x, 16: "In no way is a thing rightly said to be known, while its substance is unknown: wherefore, when the mind knows itself, it knows its substance; and when it is certain of itself, it is certain of its substance." Perhaps we may say that every mind has some limited certain knowledge as well of its own existence as of its own substance, but not an adequate knowledge of its own substance, else there would be no bad psychology.
Of God and His Creatures: 3.46