nLab epistemology


An epistemology is a certain type of cartesian bicategory that admits a “free cocompletion”.

For now it is an experimental notion, testing how far it might be possible to “do” enriched category theory (including working with enriched functor categories) without regard to size considerations. To what extent they can be said to “exist” is part of some ongoing investigation, but we give the axioms below.


Recall that a proarrow equipment consists of

  • a (weak) 2-category (or bicategory) MM, generally thought of as a bicategory of modules,

  • a 2-category KK together with a functor (a homomorphism of bicategories in the older terminology) i:KMi: K \to M that is locally fully faithful and bijective on objects, for which i(f)i(f) is a left adjoint 1-cell in MM for each 1-cell f:abf: a \to b in KK. This i(f)i(f) is often denoted B(1,f):abB(1, f): a \nrightarrow b, with right adjoint denoted B(f,1):baB(f, 1): b \nrightarrow a.

We say that the proarrow equipment is potent if i:KMi: K \to M admits a right (bi-)adjoint functor p:MKp: M \to K that is KZ (Kock-Zöberlein), meaning that the induced 2-monad pi:KKp i: K \to K is lax-idempotent. This 2-monad is to be thought of as a free cocompletion.

Another way of stating the lax-idempotence: let y:1 Kpiy: 1_K \to p i denote the unit and e:ip1 Me: i p \to 1_M the counit of the 2-monad. In addition we have triangulator isomorphisms t:1 i(ei)(iy)t: 1_i \stackrel{\sim}{\to} (e i)(i y) and s:(pe)(yp)1 ps: (p e)(y p) \stackrel{\sim}{\to} 1_p as in

ipi iy t ei i 1 i i\array{ & & i p i & & \\ & \mathllap{i y} \nearrow & \Uparrow t & \searrow \mathrlap{e i} & \\ i & & \underset{1_i}{ \; \; \; \; \; \to \; \; \; \; \; \; \; } & & i }
p 1 p p yp s pe pip \array{ p & & \overset{1_p}{ \; \; \; \; \; \to \; \; \; \; \; \; \; } & & p \\ & \mathllap{y p} \searrow & \Uparrow s & \nearrow \mathrlap{p e} \\ & & p i p & & }

and the adjunction is KZ if tt is the unit of an adjunction iyeii y \dashv e i (in other words, e=B(y,1)e = B(y, 1) according to the notion of proarrow equipment). This is equivalent to the condition that ss is the counit of an adjunction peypp e \dashv y p in KK.

Created on May 9, 2016 at 17:47:28. See the history of this page for a list of all contributions to it.