Theorem. Löb induction for presheaves on a direct category
Let be a direct category and a presheaf on it. Every map
has a fixed point: a global element with
and this fixed point is unique.
The hypothesis says: the value of at any object is determined by its values over the strict past — turns a coherent family over the strict downset of into a value at . The proof is recursion along the well-founded : at each , the section already constructed over the past assembles into an element of , and extends it to .