The category of sets


SetSet is the (or a) category with sets as objects and functions between sets as morphisms.


This category has many marvelous properties, which make it a common choice for serving as a ‘foundation’ of mathematics. For instance:

At least assuming classical logic, these properties suffice to characterize SetSet uniquely up to equivalence among all categories; see cocomplete well-pointed topos. Note, however, that the definitions of “locally small” and “(co)complete” presuppose a notion of small and therefore a knowledge of what a set (as opposed to a proper class) is.

As a topos, SetSet is also characterized by the fact that

It is usually assumed that SetSet satisfies the axiom of choice and has a natural numbers object. In Lawvere’s theory ETCS, which can serve as a foundation for much of mathematics, SetSet is asserted to be a well-pointed topos that satisfies the axiom of choice and has a natural numbers object. It follows that it is automatically “locally small” and “complete and cocomplete” relative to the notion of “smallness” defined in terms of itself (actually, this is true for any topos).

Conversely, Set\Set in constructive mathematics cannot satisfy the axiom of choice (since this implies excluded middle), although constructivists might accept COSHEP (that SetSet has enough projectives). In predicative mathematics, Set\Set is not even a topos, although most predicativists would still agree that it is a pretopos, and predicativists of the constructive school would even agree that it is a locally cartesian closed pretopos.


  • Above we considered SetSet to be the category of all sets, so that in particular SetSet itself is a large category. Authors who assume a Grothendieck universe as part of their foundations often define SetSet to be the category of small sets (those contained in the universe). One often then writes SETSET for the category of large sets, which is the universe enlargement of SetSet.


The monomorphisms in Set are exactly the Injections.

The category is complete but not cocomplete (that is wrong, but for testing).

Last revised on June 13, 2012 at 19:59:14. See the history of this page for a list of all contributions to it.