Remark 3.1.2.6. Let $f: X_{} \rightarrow S_{}$ be a morphism of simplicial sets. The following conditions are equivalent:

- $(a)$
The morphism $f$ is a Kan fibration (Definition 3.1.1.1).

- $(b)$
For every square diagram of simplicial sets

\[ \xymatrix@R =50pt@C=50pt{ A_{} \ar [d]^{i} \ar [r] & X_{} \ar [d]^{f} \\ B_{} \ar [r] \ar@ {-->}[ur] & S_{} } \]where $i$ is anodyne, there exists a dotted arrow rendering the diagram commutative.

The implication $(b) \Rightarrow (a)$ is immediate from the definitions (since the horn inclusions $\Lambda ^{n}_{i} \hookrightarrow \Delta ^ n$ are anodyne for $n > 0$). The reverse implication follows from the fact that the collection of those morphisms of simplicial sets $i: A_{} \rightarrow B_{}$ which have the left lifting property with respect to $f$ is weakly saturated (Proposition 1.4.4.16).