The functor category of contravariant set-valued functors \([{\cal C}^{op}, \mathbf {Set}]\), called the category of presheaves or varying sets, the objects of which are contravariant functors \({\cal C}^{op} \to \) Set. It may be regarded as a category of diagrams in \(\mathbf {Set}\) indexed contravariantly by the objects of \({\cal C}\).
By definition, objects of \({\cal C}\) play the role of stages, marking the "positions" (in passive view) or "movements" (in active view) of the varying set \(\mathscr {F}: {\cal C}^{op} \to \mathbf {Set}\). For every \(X\) in \({\cal C}^{op}\), the set \(\mathscr {F}(X)\) is a set of elements of \(\mathscr {F}\) at stage \(X\).
An arrow \(f: Y \to X\) between two objects in \({\cal C}^{op}\) induces a transition arrow \(\mathscr {F}(f): \mathscr {F}(X) \to \mathscr {F}(Y)\) between the varying set \(\mathscr {F}\) at stage \(A\) and the varying set \(\mathscr {F}\) at stage \(B\).