A horse walks into a bar and orders a pint

The barkeep says, "You're in here pretty often. Do you think you might be an alcoholic?"

The horse replies, "I don't think I am," and vanishes from existence.

See, the joke is about Descartes' famous philosophical statement "I think, therefore I am", but to explain that part before the rest of the joke would be putting Descartes before the horse.