A horse walks into a bar and orders a pint. The barkeep says "You're in here pretty often. Think you might be an alcoholic?", to which the horse says "I don't think I am.", and vanishes from existence. You see, the joke is about Descartes' famous philosophy of "I think therefore, I am", but to explain that part before the rest of the joke would be to put Descartes before the horse.