## 4.1 Introduction

Categories were first introduced in [GenEqui]. The category of categories (which is a proper class) is a $2$-category. Similarly, the category of stacks forms a $2$-category. If you already know about categories, but not about $2$-categories you should read Section 4.28 as an introduction to the formal definitions later on.

## Post a comment

Your email address will not be published. Required fields are marked.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like `$\pi$`

). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).

Unfortunately JavaScript is disabled in your browser, so the comment preview function will not work.

All contributions are licensed under the GNU Free Documentation License.

## Comments (0)