Let $T$ be a base type theory. We work in $T$. Let $X_{1}$ and $X_{2}$ be toposes. We say a morphism $f:X_{1}\rightarrow X_{2}$ is essential if ${f}^{*}:\mathcal {S}(X_{2},0)\rightarrow \mathcal {S}(X_{1},0)$ has a left adjoint ${f}_{!}$.