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In: Mathematics
Wikipedia [helpfully gives a bunch of reasons](https://en.wikipedia.org/wiki/Factorial#Factorial_of_zero).
As with a lot of maths things, a bunch of things work better if we define things that way, it fits with existing conventions, and it kind of makes sense with the idea of what factorial tells us.
We *could* define it to be something else, but it wouldn’t be quite as neat.
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