Comment by olikas

This happens a lot in mathematics. Definitions that depend on intuition usually have very weird edgecases. When you formalise a definition and explore the definition to a “wider” domain, then the intuition usually breaks and can have surprising consequencies. It also happens a lot in calculus. Things that work beautifully to continuous smooth functions can have an a strange edge cases. Eg if you add infinitely many continuous and smooth wave functions the result can be a non continuous function. Same here, the intuitive definition of a prime, when formalised, and applied to non positive number sets have unintuitive consequencies. One is that it can actually work.