Rene Descartes walks into a bar. The Bartender says, ""Hey, Rene, you want a scotch?"" Descartes replies, ""No, I think not."" And then he vanishes. No dice.

So a horse walks into a bar... So a horse walks into a bar. The barkeep seems to know that the horse frequently goes to bars, and he asks the horse, ""Aren't you an alcoholic by now?"" The horse replies, ""I think not!"" and disappears, seemingly into thin air. See, the joke here is based on Rene Descartes' famous philosophy of ""I think, therefore I am"", however, explaining that part first would be putting Descartes before the horse.

Descartes walks into the bar. The bartender asks him, ""will you have your usual tonight?"" Rene replies ""I think not"" and he disappears.

Rene Descartes walks into a bar ~~The bartender asks him if he wants a drink.~~ ~~""I don't think so...""~~ The bartender says ""Hey, Rene, gonna have your usual?"" ""I don't think I am."" Descartes disappears. Edit: Fixed it by popular demand.