1033: Formal Logic
Title text: Note that this implies you should NOT honk solely because I stopped for a pedestrian and you're behind me.
This comic is a riff on bumper stickers that say "honk if you love ____". Here, the subject is formal logic, but the word "if" is replaced with a formal logic term "iff," which means "if and only if".
The term "If and only if" sets two separate requirements, both of which must be met. In this case, you must love formal logic in order to be allowed to honk, and you must honk if you love formal logic. (Conversely, someone who does not love formal logic is prohibited from honking, and someone who loves formal logic cannot refuse to honk.) The title text further elaborates on this, saying in essence: "Don't honk at me just because you're impatient that I stopped for a pedestrian."
The joke is the contained self-reference: you have to love formal logic to take the sticker seriously and honk for exclusively that reason. The title text reveals the sticker is actually there to stop people from honking at him altogether, because Randall understandably hates it when he yields for pedestrians only to get honked at by some impatient driver behind him; the ONLY reason you're allowed to honk is to declare your love for formal logic. So by extension, if someone DOES honk while he is stopped for a pedestrian, he can simply enjoy the idea that the other driver loves formal logic rather than is being impatient, transforming what might otherwise be an irritant into pleasure.
- [The rear end of a car (with an unreadable license plate), with a bumper sticker above the tailpipe to the right:]
- Honk iff you love
- formal logic
- The xkcd store sells "Formal Logic" bumper stickers similar to the one in this comic.
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