nLab logic


‘Contrariwise,’ continued Tweedledee, ‘if it was so, it might be; and if it were so, it would be; but as it isn’t, it ain’t. That’s logic.’

(Lewis Carroll, Through the Looking Glass)




The basis of it all

 Set theory

set theory

Foundational axioms

foundational axioms

Removing axioms

(0,1)(0,1)-Category 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

logicset theory (internal logic of)category theorytype theory
predicatefamily of setsdisplay morphismdependent type
proofelementgeneralized elementterm/program
cut rulecomposition of classifying morphisms / pullback of display mapssubstitution
introduction rule for implicationcounit for hom-tensor adjunctionlambda
elimination rule for implicationunit for hom-tensor adjunctionapplication
cut elimination for implicationone of the zigzag identities for hom-tensor adjunctionbeta reduction
identity elimination for implicationthe other zigzag identity for hom-tensor adjunctioneta conversion
truesingletonterminal object/(-2)-truncated objecth-level 0-type/unit type
falseempty setinitial objectempty type
proposition, truth valuesubsingletonsubterminal object/(-1)-truncated objecth-proposition, mere proposition
logical conjunctioncartesian productproductproduct type
disjunctiondisjoint union (support of)coproduct ((-1)-truncation of)sum type (bracket type of)
implicationfunction set (into subsingleton)internal hom (into subterminal object)function type (into h-proposition)
negationfunction set into empty setinternal hom into initial objectfunction type into empty type
universal quantificationindexed cartesian product (of family of subsingletons)dependent product (of family of subterminal objects)dependent product type (of family of h-propositions)
existential quantificationindexed disjoint union (support of)dependent sum ((-1)-truncation of)dependent sum type (bracket type of)
logical equivalencebijection setobject of isomorphismsequivalence type
support setsupport object/(-1)-truncationpropositional truncation/bracket type
n-image of morphism into terminal object/n-truncationn-truncation modality
equalitydiagonal function/diagonal subset/diagonal relationpath space objectidentity type/path type
completely presented setsetdiscrete object/0-truncated objecth-level 2-type/set/h-set
setset with equivalence relationinternal 0-groupoidBishop set/setoid with its pseudo-equivalence relation an actual equivalence relation
equivalence class/quotient setquotientquotient type
inductioncolimitinductive type, W-type, M-type
higher inductionhigher colimithigher inductive type
-0-truncated higher colimitquotient inductive type
coinductionlimitcoinductive type
presettype without identity types
set of truth valuessubobject classifiertype of propositions
domain of discourseuniverseobject classifiertype universe
modalityclosure operator, (idempotent) 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




Traditionally, as a discipline, logic is the study of correct methods of reasoning. Logicians have principally studied deduction, the process of passing from premises to conclusion in such a way that the truth of the former necessitates the truth of the latter. In other words, deductive logic studies what it is for an argument to be valid. A second branch of logic studies induction, reasoning about how to assess the plausibility of general propositions from observations of their instances. This has often been done in terms of probability theory, particularly Bayesian.

Some philosophers, notably Charles Peirce, considered there to be third variety of reasoning for logic to study, namely, abduction. This is a process whereby one reasons to the truth of an explanation from its ability to account for what is observed. It is therefore sometimes also known as inference to the best explanation. At least some aspects of this can also be studied using Bayesian probability.

Deductive logic is the best developed of the branches. For centuries, treatments of the syllogism were at the forefront of the discipline. In the nineteenth century, however, spurred largely by the needs of mathematics, in particular the need to handle relations and quantifiers, a new logic emerged, known today as predicate logic.

As we said above, logic is traditionally concerned with correct methods of reasoning, and philosophers (and others) have had much to say prescriptively about logic. However, one can also study logic descriptively, taking it to be the study of methods of reasoning, without attempting to determine whether these methods are correct. One may study constructive logic, or a substructural logic, without saying that it should be adopted. Also psychologists study how people actually reason rapidly in situations without full information, such as by the fast and frugal approach.

A logic is a specific method of reasoning. There are several ways to formalise a logic as a mathematical object; see at Mathematical Logic below.

Mathematical logic

Mathematical logic or symbolic logic is the study of logic and foundations of mathematics as, or via, formal systems – theories – such as first-order logic or type theory.

Classical subfields

The classical subfields of mathematical logic are

Categorical logic

By a convergence and unification of concepts that has been named computational trinitarianism, mathematical logic is equivalently incarnated in

  1. type theory

  2. category theory

  3. programming theory

The logical theory that is specified by and specifies a given category 𝒞\mathcal{C} – called its internal logic, see there for more details and also see internal language, syntactic category. – is the one

Hence pure mathematical logic in the sense of the study of propositions is identified with (0,1)-category theory: where one concentrates only on (-1)-truncated objects. Genuine category theory, which is about 0-truncated objects, is the home for logic and set theory, or rather type theory, the 0-truncated objects being the sets/types/h-sets.

For instance,

Generally, (∞,1)-category theory, which is about untruncated objects, is the home for logic and types with a constructive notion of equality, the identity types in homotopy type theory.

See also at categorical model theory.

Entries on logic

\phantom{-}symbol\phantom{-}\phantom{-}in logic\phantom{-}
A\phantom{A}\inA\phantom{A}element relation
A\phantom{A}:\,:A\phantom{A}typing relation
A\phantom{A}\vdashA\phantom{A}A\phantom{A}entailment / sequentA\phantom{A}
A\phantom{A}\topA\phantom{A}A\phantom{A}true / topA\phantom{A}
A\phantom{A}\botA\phantom{A}A\phantom{A}false / bottomA\phantom{A}
A\phantom{A}\LeftrightarrowA\phantom{A}logical equivalence
A\phantom{A}\neqA\phantom{A}negation of equality / apartnessA\phantom{A}
A\phantom{A}\notinA\phantom{A}negation of element relation A\phantom{A}
A\phantom{A}¬¬\not \notA\phantom{A}negation of negationA\phantom{A}
A\phantom{A}\existsA\phantom{A}existential quantificationA\phantom{A}
A\phantom{A}\forallA\phantom{A}universal quantificationA\phantom{A}
A\phantom{A}\wedgeA\phantom{A}logical conjunction
A\phantom{A}\veeA\phantom{A}logical disjunction
symbolin type theory (propositions as types)
A\phantom{A}\toA\phantom{A}function type (implication)
A\phantom{A}×\timesA\phantom{A}product type (conjunction)
A\phantom{A}++A\phantom{A}sum type (disjunction)
A\phantom{A}00A\phantom{A}empty type (false)
A\phantom{A}11A\phantom{A}unit type (true)
A\phantom{A}==A\phantom{A}identity type (equality)
A\phantom{A}\simeqA\phantom{A}equivalence of types (logical equivalence)
A\phantom{A}\sumA\phantom{A}dependent sum type (existential quantifier)
A\phantom{A}\prodA\phantom{A}dependent product type (universal quantifier)
symbolin linear logic
A\phantom{A}\multimapA\phantom{A}A\phantom{A}linear implicationA\phantom{A}
A\phantom{A}\otimesA\phantom{A}A\phantom{A}multiplicative conjunctionA\phantom{A}
A\phantom{A}\oplusA\phantom{A}A\phantom{A}additive disjunctionA\phantom{A}
A\phantom{A}&\&A\phantom{A}A\phantom{A}additive conjunctionA\phantom{A}
A\phantom{A}\invampA\phantom{A}A\phantom{A}multiplicative disjunctionA\phantom{A}
A\phantom{A}!\;!A\phantom{A}A\phantom{A}exponential conjunctionA\phantom{A}



For centuries, logic was Aristotle's logic of deduction by syllogism. In the 19th century the idea of objective logic as metaphysics was influential

This “old logic” was famously criticized

as opposed to the “new logic” of Peano and Frege, contemporary predicate logic.

Textbooks on mathematical logic:

On categorical logic

  • Michael Makkai and Gonzalo Reyes, First Order Categorical Logic: Model-theoretical methods in the theory of topoi and related categories, Lecture Notes in Math. 611, Springer-Verlag, 1977. re-written by Francisco Marmolejo in 2018 (web announcement, pdf)

Logic in natural languages

  • Pieter A. M. Seuren, The logic of language, vol. II of Language from within; (vol. I: Language in cognition) Oxford University Press 2010

Last revised on January 4, 2024 at 06:19:59. See the history of this page for a list of all contributions to it.