Descartes: The most primary, certain, and indubitable thing I know is this: I think; I exist.
Aquinas: And how is this so?
Descartes: Because I cannot know with certainty that anything else exists. If there are no heavens, no bodies, no minds, and I am being deceived by something more powerful than myself, it is still true that I think; and there must be an "I" which is thinking. Even if I were being deceived, it is still "I" which is the object of deception, and it can never be brought about that I am nothing so long as I think that I am something. I think, I exist.
Aquinas: If you are being deceived, though, why is it that you cannot be deceived into thinking that you exist when you really do not?
Descartes: Because I am still thinking, and it cannot be that I think but I do not exist.
Aquinas: Because if you are thinking then there must be a "you" which is doing the thinking?
Aquinas: So it is implicit within the statement "I think," that you must exist.
Descartes: I must say, you are much more intelligent than I ever thought. You have caught on so quickly. You understand my first, most primary, most certain, and indubitable principle so well.
Aquinas: Thank you, Rene. But, if I may, I have one more question to ask you.
Descartes: By all means, ask.
Aquinas: Why is it that you are not being deceived right now into thinking that your principle is true? Why is it that you know that you must exist because your thinking presupposes your existence?
Descartes: What do you mean? Are you saying that it is possible that I do and do not exist right now? That is absolutely ridiculous!
Descartes: Because nothing can both be and not be at the same time and in the same manner.
Aquinas: So you know you exist now because you knew before that it must either be one or the other. You knew that you either existed or you did not, and you never thought that it might be both.
Aquinas: So there is something that you claim to know before you know whether or not you exist, namely, that nothing can both be and not be at the same time and in the same way?
Descartes: Well, yes.
Aquinas: So it seems that we have an even more certain, more primary, and more indubitable premise than your original one.
Descartes: It appears so. Well, what kind of philosophy do we get if we begin with this more known premise instead of my less known one?