It's a joke that is meant to both criticise category theorists and ring theorests (elsewhere on HN someone mentioned these kind of jokes have a name). But I do

It's a joke that is meant to both criticise category theorists and ring theorests (elsewhere on HN someone mentioned these kind of jokes have a name). But I don't think anyone here understood the joke. For a category theorest N and N_0 are equivalent. You have an initial number and a successor function, that's it. For a concrete mathematician, such as a ring theorist, it is much more important whether it's a natural number or not. They ask questions about numbers and how you can study properties of numbers (like number theorists). So my point was that if you find a category theorist insisting that 1 is a natural number and that 0 is not, then they don't view it as a set with an initial number and successor function. This would be somewhat unusual. For that matter you can start at 42 and include all numbers > 42. Then you can just get back to N via a bijection.