The philosopher Descartes walks into a bar. As he sits at the bar, the bartender notices that Descartes is in a bad mood. Trying to cheer up his friend, he says, “Descartes, would you like a beer?”
Descartes looks up and says, “I think not.”
And then, he disappears!
If this joke doesn't make any sense to you, find a friend who took philosophy in college.