On The Holy Trinity by St. Augustine
Chapter 3.--The Image of the Trinity in the Mind of Man Who Knows Himself and Loves Himself. The Mind Knows Itself Through Itself.
3. For the mind cannot love itself, except also it know itself; for how can it love what it does not know? Or if any body says that the mind, from either general or special knowledge, believes itself of such a character as it has by experience found others to be and therefore loves itself, he speaks most foolishly. For whence does a mind know another mind, if it does not know itself? For the mind does not know other minds and not know itself, as the eye of the body sees other eyes and does not see itself; for we see bodies through the eyes of the body, because, unless we are looking into a mirror, we cannot refract and reflect the rays into themselves which shine forth through those eyes, and touch whatever we discern, -- a subject, indeed, which is treated of most subtlely and obscurely, until it be clearly demonstrated whether the fact be so, or whether it be not. But whatever is the nature of the power by which we discern through the eyes, certainly, whether it be rays or anything else, we cannot discern with the eyes that power itself; but we inquire into it with the mind, and if possible, understand even this with the mind. As the mind, then, itself gathers the knowledge of corporeal things through the senses of the body, so of incorporeal things through itself. Therefore it knows itself also through itself, since it is incorporeal; for if it does not know itself, it does not love itself.