Arithmetic is not a categorical theory, meaning there is no unique model for it: https://math.stackexchange.com/questions/4667959/are-there-nonstandard-models-o

Arithmetic is not a categorical theory, meaning there is no unique model for it: https://math.stackexchange.com/questions/4667959/are-there-nonstandard-models-of-arithmetic-that-dont-add-additional-axioms https://math.stackexchange.com/questions/4667959/are-there-n...