Descartes - I think, therefore I am. Berkeley - To be is to be perceived (his version is prettied up in fancy words with religion thrown in). Kierkegaard - To exist is to have the ability to feel angst.

Craig A. Eddy - I am, therefore I think, I am perceived, and I have the ability to feel angst. One should always start with what one knows.

Craig A. Eddy, B.A. Philosophy (which is to say a Bachelor of Arts degree in BS)