Augustine, On Free Choice of Will, Book II,
chapter III:
Augustine: So, to start off with what is clearest, I ask first whether you yourself exist. Are you perhaps afraid that you might be deceived in this line of questioning? Surely if you did not exist you could not be deceived at all.
Evodius: Go on.
Augustine: Therefore, since it is clear that you exist, and it would not be clear to you unless you were alive, this too is clear: You are alive. Do you understand that these two points are absolutely true?
Evodius: Yes indeed.
Augustine: Then this third point is also clear, namely: You understand.
Evodius: Clearly.