On The Holy Trinity by St. Augustine
Chapter 9.--The Mind Knows Itself, by the Very Act of Understanding the Precept to Know Itself.
12. Let it not therefore seek to discern itself as though absent, but take pains to discern itself as present. Nor let it take knowledge of itself as if it did not know itself, but let it distinguish itself from that which it knows to be another. For how will it take pains to obey that very precept which is given it, |Know thyself,| if it knows not either what |know| means or what |thyself| means? But if it knows both, then it knows also itself. Since |know thyself| is not so said to the mind as is |Know the cherubim and the seraphim;| for they are absent, and we believe concerning them, and according to that belief they are declared to be certain celestial powers. Nor yet again as it is said, Know the will of that man: for this it is not within our reach to perceive at all, either by sense or understanding, unless by corporeal signs actually set forth; and this in such a way that we rather believe than understand. Nor again as it is said to a man, Behold thy own face; which he can only do in a looking-glass. For even our own face itself is out of the reach of our own seeing it; because it is not there where our look can be directed. But when it is said to the mind, Know thyself; then it knows itself by that very act by which it understands the word |thyself;| and this for no other reason than that it is present to itself. But if it does not understand what is said, then certainly it does not do as it is bid to do. And therefore it is bidden to do that thing which it does do, when it understands the very precept that bids it.