Cogito Ergo Sum XIV

Rene Descartes, Search for Truth (CSM II, 411-412):

Eudoxus. You cannot deny that you have such doubts; rather it is certain that you have them, so certain in fact that you cannot doubt your doubting. Therefore it is also true that you who are doubting exist; this is so true that you can no longer have any doubts about it.

Polyander. I quite agree with you on that point, because if I did not exist, I would not be able to doubt.

Eudoxus. You exist, therefore, and you know that you exist, and you know this just because you are doubting.

Polyander. All of this is quite true.

Eudoxus. But, so that you are not deflected from the course I suggested, let us proceed gradually, and as I said, you will find that you are making greater progress than you think. Let us go through the argument again. You exist, and you know that you exist, and you know this because you know that you are doubting....