A horse walks into a bar and the bartender asks "hey, don't you think you might be an alcoholic, with all the bars you frequent?"

The horse says "I think not!" And disappears in a cloud of smoke.

Now, if you know anything about philosophy, you'll be laughing, because of the saying 'Cogito ergo sum.'

I could have explained that before the joke, but it would have been putting Descartes before the horse.