The pair $\webleft (\mathcal{P}\webleft (X\webright ),\chi _{\webleft (-\webright )}\webright )$ consisting of

  • The powerset of $X$ together with reverse inclusion $\mathcal{P}\webleft (X\webright )^{\mathsf{op}}=\webleft (\mathcal{P}\webleft (X\webright ),\supset \webright )$ of Definition 2.4.1.1.1;
  • The characteristic embedding $\chi _{\webleft (-\webright )}\colon X\hookrightarrow \mathcal{P}\webleft (X\webright )$ of $X$ into $\mathcal{P}\webleft (X\webright )$ of Definition 2.5.4.1.1;
satisfies the following universal property:

  • Given another pair $\webleft (Y,f\webright )$ consisting of
    • An inflattice $\webleft (Y,\preceq \webright )$;
    • A function $f\colon X\to Y$;
    there exists a unique morphism of inflattices

    \[ \webleft (\mathcal{P}\webleft (X\webright ),\supset \webright )\overset {\exists !}{\to }\webleft (Y,\preceq \webright ) \]

    making the diagram

    commute.

This is a rephrasing ofProposition 2.4.6.1.2, which we prove below.1


1Here we only remark that the unique morphism of inflattices in the statement is given by the right Kan extension $\text{Ran}_{\chi _{X}}\webleft (f\webright )$ of $f$ along $\chi _{X}$.


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