nLab
Cat

Idea

Cat is a name for the category or 2-category of all categories.

This is also the archetypical 2-topos.

Definition

To avoid set-theoretic problems related to Russell's paradox, it is typical to restrict $Cat$ to small categories. But see CAT for alternatives.

To be explicit, define Cat to be the category with:

• small categories as objects,

• functors as morphisms;

• composition of morphisms the evident composition of functors.

This is probably the most common meaning of $Cat$ in the literature.

We more often use Cat to stand for the strict 2-category with:

• small categories as objects,
• functors as morphisms.
• natural transformations as 2-morphisms.

Here the vertical composition of 2-morphisms is the evident composition of component maps of natural transformations, whereas the horizontal composition is given by their Godement product.

Finally, we can use Cat for the bicategory with:

• small categories as objects,
• anafunctors as morphisms.
• ananatural transformations as 2-morphisms.

To be really careful, this version of $Cat$ is an anabicategory.

Properties

Cartesian closed structure

The category $Cat$, at least in its traditional version comprising small categories only, is cartesian closed: the exponential objects are functor categories. Direct proofs can be found in:

• p. 98 of Categories Work, 2nd ed.
• remark below Definition 4.3.9 in Category Theory in Context
• Steve Awodey, Category Theory, Second Edition, Sections 7.5-7.7.

A more indirect proof could proceed by identifying $Cat$ via the nerve construction as a reflective subcategory of sSet, which is cartesian closed as it is a presheaf category, and showing that this subcategory is an exponential ideal.

Size issues

As a $2$-category, $Cat$ could even include (some) large categories without running into Russell’s paradox. More precisely, if $U$ is a Grothendieck universe such that $\Set$ is the category of all $U$-small sets, then you can define $\Cat$ to be the 2-category of all $U'$-small categories, where $U'$ is some Grothendieck universe containing $U$. That way, you have $\Set \in \Cat$ without contradiction. (This can be continued to higher categories.)

By the axiom of choice, the two definitions of $Cat$ as a $2$-category are equivalent. In contexts without choice, it is usually better to use anafunctors all along; if necessary, use $Str Cat$ for the strict $2$-category. Even without choice, a functor or anafunctor between categories is an equivalence in the anabicategory $Cat$ iff it is essentially surjective and fully faithful. However, the weak inverse of such a functor may not be a functor, so it need not be an equivalence in $Str Cat$. We can regard $Cat$ as obtained from $Str Cat$ using homotopy theory by “formally inverting” the essentially surjective and fully faithful functors as weak equivalences.

Colimits

• pushouts in Cat of injective functors are considered in (MacdonaldScull).

Related concepts

• Pos

• Set

  • Rel, Prof

• Grpd, ∞Grpd

• $Cat$,

  • elementary theory of the 2-category of categories (ETCC)

  • PrCat

  • Operad

  • VCat

• 2Cat

• (∞,1)Cat

  • Pr(∞,1)Cat

  • (∞,1)Operad

• (∞,n)Cat

References

See also the references at category and category theory.

Discussion of (certain) pushouts in $Cat$ is in

• John Macdonald, Laura Scull, Amalgamations of categories (pdf)

category: category