Cogito Ergo Sum VI

Augustine, On the Trinity, Book X, chapter 10:

But since we treat of the nature of the mind, let us remove from our consideration all knowledge which is received from without, through the senses of the body; and attend more carefully to the position which we have laid down, that all minds know and are certain concerning themselves. For men certainly have doubted whether the power of living, of remembering, of understanding, of willing, of thinking, of knowing, of judging, be of air, or of fire, or of the brain, or of the blood, or of atoms, or besides the usual four elements of a fifth kind of body, I know not what; or,whether the combining or tempering together of this our flesh itself has power to accomplish these things. And one has attempted to establish this, and another to establish that. Yet who ever doubts that he himself lives, and remembers, and understands, and wills, and thinks, and knows, and judges? Seeing that even if he doubts, he lives; if he doubts, he remembers why he doubts; if he doubts, he understands that he doubts; if he doubts, he wishes to be certain; if he doubts, he thinks; if he doubts, he knows that he does not know; if he doubts, he judges that he ought not to assent rashly. Whosoever therefore doubts about anything else, ought not to doubt of all these things; which if they were not, he would not be able to doubt of anything.