first-order theory


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


Model theory



A first-order theory is a theory written in the language of first-order logic i.e it is a set of formulas or sequents (or generally, axioms over a signature) whose quantifiers and variables range over individuals of the underlying domain, but not over subsets of individuals nor over functions or relations of individuals etc. A first-order theory is called infinitary when the expressions contain infinite disjunctions or conjunctions, else it is called finitary.

Another name for a first-order theory is elementary theory which is found in the older literature but by now has gone extinct though it has left its traces in names like elementary topos, elementary embedding, ETCC etc.

The characterization of set-theoretic models of (finitary) first-order theories is the topic of traditional model theory.


Formulations of set theory are usually first-order theories, such as


In particular, the precise formulation of the former in first-order predicate logic by Skolem in 1922 provided the impetus for the hold that first-order predicate logic gained over mathematical logic in the following.

It is somewhat ironic that classical first-order logic owes its promotion to prominence to intuitionistic mathematicians like Weyl (1910,1918) and Skolem.



Let 𝕋\mathbb{T} be a finitary first-order theory. There exists a Grothendieck topos \mathcal{E} and a 𝕋\mathbb{T}-model N 𝕋N_{\mathbb{T}} in \mathcal{E} such that the first-order sequents satisfied in N 𝕋N_{\mathbb{T}} are precisely those provable in 𝕋\mathbb{T}.

This result due to P. Freyd is discussed in Johnstone (2002, p.900) or Freyd-Scedrov (1990, p.130).

If 𝕋\mathbb{T} is a theory over a countable signature, then \mathcal{E} can be taken as the topos Sh(2 )Sh(2^{\mathbb{N}}) of sheaves on the Cantor space 2 2^{\mathbb{N}}. This completeness result shows that Sh(2 )Sh(2^{\mathbb{N}}) plays in constructive first-order logic a role similar to the role of SetSet in classical first-order logic.


Last revised on February 24, 2017 at 02:48:02. See the history of this page for a list of all contributions to it.