# Category theory

Every retraction is an epimorphism, and every section is a monomorphism. Furthermore, the following three statements are equivalent:

Functors are structure-preserving maps between categories. They can be thought of as morphisms in the category of all (small) categories.

A *natural transformation* is a relation between two functors. Functors often describe "natural constructions" and natural transformations then describe "natural homomorphisms" between two such constructions. Sometimes two quite different constructions yield "the same" result; this is expressed by a natural isomorphism between the two functors.

Using the language of category theory, many areas of mathematical study can be categorized. Categories include sets, groups and topologies.

The definitions of categories and functors provide only the very basics of categorical algebra; additional important topics are listed below. Although there are strong interrelations between all of these topics, the given order can be considered as a guideline for further reading.

Many of the above concepts, especially equivalence of categories, adjoint functor pairs, and functor categories, can be situated into the context of *higher-dimensional categories*. Briefly, if we consider a morphism between two objects as a "process taking us from one object to another", then higher-dimensional categories allow us to profitably generalize this by considering "higher-dimensional processes".

It should be observed first that the whole concept of a category is essentially an auxiliary one; our basic concepts are essentially those of a functor and of a natural transformation [...]

*The Topos of Music, Geometric Logic of Concepts, Theory, and Performance*