Example 6.78.
Every étale morphism of topoi is essential: given a topos \(T\) and an object \(U \in T\), the functor \(j^*\colon T \to T_{/U}\) given by \(X \mapsto X \times U\) admits a left adjoint \(j_\sharp\colon T_{/U} \to T\) given by the forgetful functor.