The category of sets

Definition

$\mathrm{Set}$ is the (or a) category with sets as objects and functions between sets as morphisms.

This definition is somewhat vague by design. Rather than canonize a fixed set of principles, the nLab adopts a ‘pluralist’ point of view which recognizes different needs and foundational assumptions among mathematicians who use set theory. Thus, there are various axes to consider when formulating categorical properties one thinks $\mathrm{Set}$ should satisfy, including

• First-order vs. higher-order logic

• Impredicative vs. predicative mathematics

• ‘Variability’ vs. ‘constancy’ (Lawvere)

• Classical logic vs. intuitionistic logic

• Choice principles

• Replacement or collection principles

• Smallness structures

just to name a few. Quite a bit of axiomatic fine-tuning can enter when one considers the panoply of hypotheses that might appeal to one or another school of intuitionism or constructivism, or various combinatorial or cardinal hypotheses one might attach to ZFC, etc.

Properties

Characterization

The category $\mathrm{Set}$ has many marvelous properties, which make it a common choice for serving as a ‘foundation’ of mathematics. For instance:

• It is a well-pointed topos,

  So in particular it is locally cartesian closed.

• It is locally small.

• It is complete and cocomplete, and therefore $\mathrm{\infty }$-extensive (as is any cocomplete topos).

At least assuming classical logic, these properties suffice to characterize $\mathrm{Set}$ 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, $\mathrm{Set}$ is also characterized by the fact that

• $\mathrm{Set}$ is the terminal object in the category of Grothendieck toposes and geometric morphisms. The terminal morphism $\Gamma :E\to \mathrm{Set}$ from any other topos $E$ is the global sections functor.

It is usually assumed that $\mathrm{Set}$ 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, $\mathrm{Set}$ 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, $Set$ in constructive mathematics cannot satisfy the axiom of choice (since this implies excluded middle), although constructivists might accept COSHEP (that $\mathrm{Set}$ has enough projectives). In predicative mathematics, $Set$ 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.

Size

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

Opposite category and Boolean algebras

See for instance van Oosten, theorem 2.4

Related categories

A variant of $\mathrm{Set}$ where functions are generalized to relations is Rel.

In higher category theory the role of $\mathrm{Set}$ is played for instance by

• Pos

• Grpd, ∞Grpd

• Cat, (∞,1)Cat

• (∞,n)Cat

References

• Jaap van Oosten, Basic category theory (pdf)