I think not

Usually used to mean "I disagree", but sounds snootier. It leaves one open to the rejoinder, "Yes, I know you think not. That's your problem."

This phrase is essential to the punchline of the only philosophy joke anyone knows. If you don't know it, this is a joke about Rene Descartes, which can be found in toto in his node. Many people tell this joke when trying to make conversation with a philosopher. This is a bad idea, not only because they've all heard it before, but because it's an excellent example of denying the antecedent. Tell it in a group of philosophers, and at least half of them will exclaim "denying the antecedent!" before you finish the joke. In short, it will blow up in your face.

Thinking is a sufficient condition for existence, but you don't have to meet too many people before you realize it isn't actually necessary.