Bad logic makes bad humor.

While I appreciate Alanna's brand of fun and humor, and enjoy her comments on my blog, I simply cannot allow the awful joke she related in the comments to this post to go unchallenged. I've always hated that joke. I don't hate it because it's a bad joke, I hate it because it's bad logic. (I also think it's a bad joke because it doesn't make sense, because it's bad logic.) Here's the "joke": Descartes is sitting at a bar. The bartender asks him if he wants another drink. He says, "I think not." Then he disappears. The background is the famous cogito: Descartes proved to himself that he existed by reasoning, I think, therefore I am. Presumably, the alleged joke is trying to do a kind of reverse modus tollens, resulting in this argument:

1. If I think, then I am.
2. I think not. Therefore,
3. I do not exist.

The problem with this argument is that it's not valid. We can't go from if A then B and not A to not B. We can go from if A then B and A to B (modus ponens), or from if A then B and not B to not A (modus tollens), but the step the "joke" relies on just doesn't work. To illustrate, there are lots of examples of things that do not think but nevertheless exist. My spiced pumpkin candle, for instance. The following are recastings of the joke which are based on good logic. I recommend that if you ever feel the need to tell a Descartes-at-a-bar joke, you tell one of these: MP: Descartes is sitting at a bar, and the bartender asks him if he'll have another drink. "Yes, I think I will," says Descartes. Suddenly, he continues to exist. MT: Descartes is sitting at a bar, and the bartender asks him if he'll have another drink. Descartes suddenly disappears. "Well, I guess I'll take that as an 'I think not'," says the bartender. AC: Descartes is sitting at a bar, and the bartender asks him if he wants another drink. "Well, I do but I don't," says Descartes. Suddenly, everything happens. W: Descartes is sitting at a bar, and the bartender asks him if needs another drink, and tells the assistant bartender that she must, if Descartes says yes, get him one. Suddenly, Descartes becomes very, very thirsty.