A horse walks into a bar. The bartender asks "hey, why the long face? Are you depressed?" The horse ponders for a second, scratches his chin, and says "I don't think I am" and promptly disappears. You see, this is a joke about Rene Descartes' famous statement "I think, therefore I am" and I could have mentioned this at the start of the joke, but that would be putting Descartes before the horse.