inhabited set


Type theory

natural deduction metalanguage, practical foundations

  1. type formation rule
  2. term introduction rule
  3. term elimination rule
  4. computation rule

type theory (dependent, intensional, observational type theory, homotopy type theory)

syntax object language

computational trinitarianism = propositions as types +programs as proofs +relation type theory/category theory

logiccategory theorytype theory
trueterminal object/(-2)-truncated objecth-level 0-type/unit type

falseinitial objectempty type

proposition(-1)-truncated objecth-proposition, mere proposition

proofgeneralized elementprogram

cut rulecomposition of classifying morphisms / pullback of display mapssubstitution

cut elimination for implicationcounit for hom-tensor adjunctionbeta reduction

introduction rule for implicationunit for hom-tensor adjunctioneta conversion

logical conjunctionproductproduct type

disjunctioncoproduct ((-1)-truncation of)sum type (bracket type of)

implicationinternal homfunction type

negationinternal hom into initial objectfunction type into empty type

universal quantificationdependent productdependent product type

existential quantificationdependent sum ((-1)-truncation of)dependent sum type (bracket type of)

equivalencepath space objectidentity type

equivalence classquotientquotient type

inductioncolimitinductive type, W-type, M-type

higher inductionhigher colimithigher inductive type

completely presented setdiscrete object/0-truncated objecth-level 2-type/preset/h-set

setinternal 0-groupoidBishop set/setoid

universeobject classifiertype of types

modalityclosure operator, (idemponent) monadmodal type theory, monad (in computer science)

linear logic(symmetric, closed) monoidal categorylinear type theory/quantum computation

proof netstring diagramquantum circuit

(absence of) contraction rule(absence of) diagonalno-cloning theorem

synthetic mathematicsdomain specific embedded programming language


homotopy levels




A set or type is inhabited if it contains an element or term.


In set theory

In set theory, an inhabited set is a set that contains an element, i.e. a set XX such that x,xX\exists x, x\in X is true.

At least assuming classical logic, this is the same thing as a set that is not empty. Usually inhabited sets are simply called ‘non-empty’, but the positive word ‘inhabited’ reminds us that inhabitation is the simpler notion, which emptiness is defined as the negation of.

The term ‘inhabited’ come from constructive mathematics. In constructive mathematics (such as the internal logic of some topos or generally in type theory), a set/type that is not empty is not already necessarily inhabited. This is because double negation is nontrivial in intuitionistic logic. All the same, many constructive mathematicians use the old word ‘non-empty’ with the understanding that it really means inhabited, and write AA\neq \emptyset to mean that AA is inhabited. The latter we can interpret literally if we regard \neq as a reference to an inequality relation other than the denial inequality, such as the inequality defined for subsets by ABx((xAxB)(xBxA))A \neq B \iff \exists x ((x\in A \wedge x\notin B)\vee(x\in B\wedge x\notin A)). If we prefer to reserve \neq for the denial inequality, then we can write #\# for this stronger inequality of sets (although it is not an apartness relation), and hence A#A\#\emptyset to mean that AA is inhabited.

In type theory

In type theory there are two possible notions of inhabited type: a type XX whose propositional truncation X\Vert X \Vert has an element (or term), or a type XX that itself has an element (or term). The former is what corresponds to the above notion of inhabitedness in set theory, since X\Vert X \Vert is the propositions as types interpretation of x:X\exists x:X. The latter is more like the notion of a pointed set.

The assertion X,(XX)\forall X, (\Vert X \Vert \to X) is a mildly nonconstructive logical principle called the propositional axiom of choice?. It follows from excluded middle, but in the internal logics of some toposes, it can fail, so that these two notions of “inhabited type” really are different.

Inhabited objects

An inhabited set is the special case of an internally inhabited object in the topos Set. The two notions of inhabited type correspond to internally and externally inhabited objects (which are, respectively, those objects XX where X1X\to 1 is an epimorphism, and those which admit a global element 1X1\to X).

There is a distinction between ‘inhabited’ and ‘occupied?’ spaces in Abstract Stone Duality (which probably corresponds to something about locales, should explain that here).

Last revised on February 15, 2017 at 15:57:04. See the history of this page for a list of all contributions to it.