A couple jokes I found:

Descartes receives his first royalty check from Meditations on First Philosophy, in which he says, "I think, therefore I am." He goes to the pub to celebrate. The bartender asks if he just wants the usual. Descartes responds, "I think not!"—and promptly disappears.

An Englishman, a Scotsman, and an Irishman walk into a pub. They proceed to each buy a pint of Guinness. Just as they're about to enjoy their creamy beverage, a fly lands in each of their pints and gets stuck in the thick head. The Englishman pushes his beer away from him in disgust. The Irishman fishes the offending fly out of his beer and continues drinking it as if nothing had happened. The Scotsman, too, picks the fly out of his drink, holds it out over the beer and then starts yelling: "SPIT IT OUT! SPIT IT OUT, YOU BASTARD!"