Proposition 3.1.5.4. Let $X_{}$ and $Y_{}$ be simplicial sets, and suppose we are given a pair of morphisms $f,g: X_{} \rightarrow Y_{}$. Then:

The morphisms $f$ and $g$ are homotopic if and only if there exists a sequence of morphisms $f = f_0, f_1, \ldots , f_ n = g$ from $X_{}$ to $Y_{}$ having the property that, for each $1 \leq i \leq n$, either there exists a homotopy from $f_{i-1}$ to $f_{i}$ or a homotopy from $f_{i}$ to $f_{i-1}$.

Suppose that $Y_{}$ is a Kan complex. Then $f$ and $g$ are homotopic if and only if there exists a homotopy from $f$ to $g$.