Since $\mathbb{P}(F) > 0$, then $F \neq \emptyset$. If $F = \Omega$, then result
R4338: Conditional probability of almost surely true event yields the claim, so we may further assume that $F \neq \Omega$. Under this assumption, the pair $F, \Omega \setminus F$ is a disjoint partition of $\Omega$ and applying results
we have
\begin{equation}
\begin{split}
\frac{\mathbb{P}(E \cap F)}{\mathbb{P}(F)}
= \frac{\mathbb{E}(I_{E \cap F})}{\mathbb{P}(F)}
& = \frac{\mathbb{E}(I_E I_F)}{\mathbb{P}(F)} \\
& = \mathbb{E}(I_E \mid \sigma \langle F \rangle)
& = \mathbb{P}(E \mid \sigma \langle F \rangle)
= \mathbb{P}(E \mid F)
\end{split}
\end{equation}
where the last two equalities hold by definition. This is what was required to be shown. $\square$