One of the milestones of contemporary philosophy of mathematics is Benacerraf‘s 1965 article “What Numbers Could Not Be“.^{1} There he offers a compelling argument according to which numbers cannot be considered as sets — namely, they cannot be *metaphysically **identified* with sets.

The argument is quite simple: if numbers were sets, we should be able to find a unique progression of sets with which numbers can be identified. But this is apparently impossible: there is a lot of ω-series that can serve as well for the aim. For example, we can adopt von Neumann’s series, and say that , and so on, where the successor function is defined by . Or we can adopt Zermelo’s series, and say that , and so on, where the successor function is defined by . Now, the problem is: is or is ? Benacerraf presents then the example of two children, Ernie and John. The first learned that von Neumann’s ordinals are the natural numbers, while the latter that Zermelo’s ordinals are the natural numbers. Now, they will be easily able to learn arithmetic set theoretically via the above constructions, and they will agree on any arithmetical theorem, except that for Ernie it is true that , while for John it is false!