Version 1

Descartes walks into a bar and sits down. The bartender asks “You wanna beer?”

Descartes says “I think not.”

…and disappears.

Version 2

A horse walks into a bar.  The bartender says, “you’re in here a lot, are you an alcoholic?”

The horse ponders for a minute and responds ” don’t think I am” and *POOF* he disappears.

This is where philosophy students start to snicker, as they are familiar with Descartes and the postulate “I think, therefore I am.”

But telling you that first would be putting Descartes before the horse