Remark 3.1.1.7. Let $f: X \rightarrow S$ be a map of simplicial sets. Suppose that, for every simplex $\sigma : \Delta ^ n \rightarrow S$, the projection map $\Delta ^ n \times _{S} X \rightarrow \Delta ^ n$ is a Kan fibration. Then $f$ is a Kan fibration. Consequently, if we are given a pullback diagram of simplicial sets
\[ \xymatrix@R =50pt@C=50pt{ X'_{} \ar [d]^{f'} \ar [r] & X_{} \ar [d]^{f} \\ S'_{} \ar [r]^-{g} & S_{} } \]
where $g$ is surjective and $f'$ is a Kan fibration, then $f$ is also a Kan fibration.