• Naturality. The collection
    \[ \webleft\{ \chi _{\webleft (-\webright )}\colon \mathcal{P}\webleft (X\webright )\to \mathsf{Sets}\webleft (X,\{ \mathsf{t},\mathsf{f}\} \webright )\webright\} _{X\in \text{Obj}\webleft (\mathsf{Sets}\webright )} \]

    defines a natural isomorphism between $\mathcal{P}^{-1}$ and $\mathsf{Sets}\webleft (-,\{ \mathsf{t},\mathsf{f}\} \webright )$. In particular, given a function $f\colon X\to Y$, the diagram

    commutes, i.e. we have

    \[ \chi _{V}\circ f=\chi _{f^{-1}\webleft (V\webright )} \]

    for each $V\in \mathcal{P}\webleft (Y\webright )$.


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