1.3 $\infty $Categories
In §1.1 and §1.2, we considered two closely related conditions on a simplicial set $S_{\bullet }$:
 $(\ast )$
For $0 \leq i \leq n$, every map of simplicial sets $\sigma _0: \Lambda ^{n}_{i} \rightarrow S_{\bullet }$ can be extended to a map $\sigma : \Delta ^{n} \rightarrow S_{\bullet }$.
 $(\ast ')$
For $0 < i < n$, every map of simplicial sets $\sigma _0: \Lambda ^{n}_{i} \rightarrow S_{\bullet }$ can be extended uniquely to a map $\sigma : \Delta ^{n} \rightarrow S_{\bullet }$.
Simplicial sets satisfying $(\ast )$ are called Kan complexes and form the basis for a combinatorial approach to homotopy theory, while simplicial sets satisfying $(\ast ')$ can be identified with categories (Propositions 1.2.2.1 and 1.2.3.1). These notions admit a common generalization:
Definition 1.3.0.1. An $\infty $category is a simplicial set $S_{\bullet }$ which satisfies the following condition:
 $(\ast '')$
For $0 < i < n$, every map of simplicial sets $\sigma _0: \Lambda ^{n}_{i} \rightarrow S_{\bullet }$ can be extended to a map $\sigma : \Delta ^{n} \rightarrow S_{\bullet }$.
Example 1.3.0.3. Every Kan complex is an $\infty $category. In particular, if $X$ is a topological space, then the singular simplicial set $\operatorname{Sing}_{\bullet }(X)$ is an $\infty $category.
Example 1.3.0.4. For every category $\operatorname{\mathcal{C}}$, the nerve $\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$ is an $\infty $category.
Throughout this book, we will generally use calligraphic letters (like $\operatorname{\mathcal{C}}$, $\operatorname{\mathcal{D}}$, and $\operatorname{\mathcal{E}}$) to denote $\infty $categories, and we will generally describe them using terminology borrowed from category theory. For example, if $\operatorname{\mathcal{C}}= S_{\bullet }$ is an $\infty $category, then we will refer to vertices of the simplicial set $S_{\bullet }$ as objects of the $\infty $category $\operatorname{\mathcal{C}}$, and to edges of the simplicial set $S_{\bullet }$ as morphisms of the $\infty $category $\operatorname{\mathcal{C}}$ (see §1.3.1). One of the central themes of this book is that $\infty $categories behave much like ordinary categories. In particular, for any $\infty $category $\operatorname{\mathcal{C}}$, there is notion of composition for morphisms of $\operatorname{\mathcal{C}}$, which we study in §1.3.4. Given a pair of morphisms $f: X \rightarrow Y$ and $g: Y \rightarrow Z$ in $\operatorname{\mathcal{C}}$ (corresponding to edges $f,g \in S_{1}$ satisfying $d_0(f) = d_1(g)$), the pair $(f,g)$ defines a map of simplicial sets $\sigma _0: \Lambda ^{2}_{1} \rightarrow \operatorname{\mathcal{C}}$. Applying condition $(\ast '')$, we can extend $\sigma _0$ to a $2$simplex $\sigma $ of $\operatorname{\mathcal{C}}$, which we can think of heuristically as a commutative diagram
\[ \xymatrix { & Y \ar [dr]^{g} & \\ X \ar [ur]^{f} \ar@ {>}[rr]^{h} & & Z. } \]
In this case, we will refer to the morphism $h = d_1(\sigma )$ as a composition of $f$ and $g$. However, this comes with a caveat: the extension $\sigma $ is usually not unique, so the morphism $h$ is not completely determined by $f$ and $g$. However, we will show that it is unique up to a certain notion of homotopy which we study in §1.3.3. We apply this observation in §1.3.5 to extract an ordinary category $\mathrm{h} \mathit{\operatorname{\mathcal{C}}}$ called the homotopy category of $\operatorname{\mathcal{C}}$, whose morphisms are homotopy classes of morphisms of $\operatorname{\mathcal{C}}$ (Definition 1.3.5.3). This is a special case of a more general construction which can be applied to any simplicial set $S_{\bullet }$, which we describe in §1.3.6.
Structure

Subsection 1.3.1: Objects and Morphisms

Subsection 1.3.2: The Opposite of an $\infty $Category

Subsection 1.3.3: Homotopies of Morphisms

Subsection 1.3.4: Composition of Morphisms

Subsection 1.3.5: The Homotopy Category

Subsection 1.3.6: The Universal Property of $\mathrm{h} \mathit{\operatorname{\mathcal{C}}}$

Subsection 1.3.7: Equivalences