Given theories like evolution and the like, I’d like to say the thing that makes me who I am is rather who had sex with who, than who was thinking or not… therefore:

Coitus ergo sum

(From latin, “I have sex, therefore I am”,  a bastardization – I love those! – of “Cognito ergo sum”, also latin, “I think, therefore I am”)

Not to mention that this is, in my humble opinion, a more honest reflection of the human condition. I mean, who stands around thinking when they could be having sex? 😛