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Created by Mario Carneiro

Intuitionistic Logic Proof Explorer

Intuitionistic Logic (Wikipedia [accessed 19-Jul-2015], Stanford Encyclopedia of Philosophy [accessed 19-Jul-2015]) can be thought of as a constructive logic in which we must build and exhibit concrete examples of objects before we can accept their existence. Unproved statements in intuitionistic logic are not given an intermediate truth value, instead, they remain of unknown truth value until they are either proved or disproved. Intuitionist logic can also be thought of as a weakening of classical logic such that the law of excluded middle (LEM), (φ ¬ φ), doesn't always hold. Specifically, it holds if we have a proof for φ or we have a proof for ¬ φ, but it doesn't necessarily hold if we don't have a proof of either one. There is also no rule for double negation elimination. Brouwer observed in 1908 that LEM was abstracted from finite situations, then extended without justification to statements about infinite collections.

Contents of this page
  • Overview of intuitionistic logic
  • Overview of this work
  • The axioms
  • Some theorems
  • Bibliography
  • Related pages
  • Table of Contents and Theorem List
  • Bibliographic Cross-Reference
  • Definition List
  • ASCII Equivalents for Text-Only Browsers
  • Metamath database (ASCII file)
  • External links
  • Github repository [accessed 06-Jan-2018]

  • Overview of intuitionistic logic

    (Placeholder for future use)

    Overview of this work

    (By Gérard Lang, 12-Feb-2018)

    Mario Carneiro's work (Metamath database) "" provides in Metamath a development of "" whose eventual aim is to show how many of the theorems of set theory and mathematics that can be derived from classical first order logic can also be derived from a weaker system called "intuitionistic logic." To achieve this task, adds (or substitutes) 12 intuitionistic axioms whose second part of the name begins with the letter "i" to the classical logical axioms of

    Among these 12 new axioms, the 6 first (ax-ia1, ax-ia2, ax-ia3, ax-io, ax-in1 and ax-in2), when substituted to ax-3 and added with ax-1, ax-2 and ax-mp, allow for the development of intuitionistic propositional logic. Each of them is a theorem of classical propositional logic, but ax-3 cannot be derived from them. Similarly, other basic classical theorems, like the third middle excluded or the equvalence of a proposition with its double negation, cannot be derived in intuitionistic propositional calculus. Glivenko showed that a proposition φ is a theorem of classical propositional calculus if and only if ¬¬φ is a theorem of intuitionistic propositional calculus.

    The next 4 new axioms (ax-ial, ax-i5r, ax-ie1 and ax-ie2) together with the axioms ax-4, ax-5, ax-7 and ax-gen do not mention equality or distinct variables.

    The last two new axioms (ax-i9 and ax-i12) are variants of the classical axioms ax-9 and ax-12. The substitution of ax-i9 and ax-i12 with ax-9 and ax-12 and the inclusion of ax-8, ax-10, ax-11, ax-13, ax-14 and ax-17 allow for the development of the intuitionistic predicate calculus.

    Each of the 12 new axioms is a theorem of classical first order logic with equality. But some axioms of classical first order logic with equality, like ax-3, cannot be derived in the intuitionistic predicate calculus.

    One of the major interests of the intuitionistic predicate calculus is that its use can be considered as a realization of the program of the constructivist philosophical view of mathematics.

    The axioms

    As with the classical axioms we have propositional logic and predicate logic.

    The axioms of intuitionistic propositional logic consist of some of the axioms from classical propositional logic, plus additional axioms for the operation of the 'and', 'or' and 'not' connectives.

    Axioms of intuitionistic propositional calculus
    Axiom Simp ax-1 (φ → (ψφ))
    Axiom Frege ax-2 ((φ → (ψχ)) → ((φψ) → (φχ)))
    Rule of Modus Ponens ax-mp φ   &   φψ   =>   ψ
    Left 'and' eliminationax-ia1 ((φ ψ) → φ)
    Right 'and' eliminationax-ia2 ((φ ψ) → ψ)
    'And' introductionax-ia3 (φ → (ψ → (φ ψ)))
    Definition of 'or'ax-io (((φ χ) → ψ) ↔ ((φψ) (χψ)))
    'Not' introductionax-in1 ((φ → ¬ φ) → ¬ φ)
    'Not' eliminationax-in2 φ → (φψ))

    Unlike in classical propositional logic, 'and' and 'or' are not readily defined in terms of implication and 'not'. In particular, φψ is not equivalent to ¬ φψ, nor is φψ equivalent to ¬ φψ, nor is φψ equivalent to ¬ (φ → ¬ ψ).

    The ax-in1 axiom is a form of proof by contradiction which does hold intuitionistically. That is, if φ implies a contradiction (such as its own negation), then one can conclude ¬ φ. By contrast, assuming ¬ φ and then deriving a contradiction only serves to prove ¬ ¬ φ, which in intuitionistic logic is not the same as φ.

    The biconditional can be defined as the conjunction of two implications, as in dfbi2 and df-bi. Other ways of understanding the biconditional, such as dfbi1 or dfbi3, however, are classical rather than intuitionistic results (following the proofs on those pages shows they depend on ax-3).

    Predicate logic adds set variables (individual variables) and the ability to quantify them with ∀ (for-all) and ∃ (there-exists). Unlike in classical logic, ∃ cannot be defined in terms of ∀. As in classical logic, we also add = for equality (which is key to how we handle substitution in metamath) and ∈ (which for current purposes can just be thought of as an arbitrary predicate, but which will later come to mean set membership).

    Our axioms are based on the classical predicate logic axioms, but adapted for intuitionistic logic, chiefly by adding additional axioms for ∃ and also changing some aspects of how we handle negations.

    Axioms of intuitionistic predicate logic
    Axiom of Specialization ax-4 (xφφ)
    Axiom of Quantified Implication ax-5 (x(φψ) → (xφxψ))
    The converse of ax-5o ax-i5r ((xφxψ) → x(xφψ))
    Axiom of Quantifier Commutation ax-7 (xyφyxφ)
    Rule of Generalization ax-gen φ   =>    xφ
    x is bound in xφ ax-ial (xφxxφ)
    x is bound in xφ ax-ie1 (xφxxφ)
    Define existential quantification ax-ie2 (x(ψxψ) → (x(φψ) ↔ (xφψ)))
    Axiom of Equality ax-8 (x = y → (x = zy = z))
    Axiom of Existence ax-i9 x x = y
    Axiom of Quantifier Substitution ax-10 (x x = yy y = x)
    Axiom of Variable Substitution ax-11 (x = y → (yφx(x = yφ)))
    Axiom of Quantifier Introduction ax-i12 (z z = x (z z = y z(x = yz x = y)))
    Axiom of Bundling ax-bnd (z z = x (z z = y xz(x = yz x = y)))
    Left Membership Equality ax-13 (x = y → (x zy z))
    Right Membership Equality ax-14 (x = y → (z xz y))
    Distinctness ax-17 (φxφ), where x does not occur in φ

    Some theorems

    (Placeholder for future use)

    1. [Bauer] Bauer, Andrej, "Five stages of accepting constructive mathematics," Bulletin (New Series) of the American Mathematical Society, 54:481-498 (2017), DOI: 10.1090/bull/1556 .
    2. [BellMachover] Bell, J. L., and M. Machover, A Course in Mathematical Logic, North-Holland, Amsterdam (1977) [QA9.B3953].
    3. [Hamilton] Hamilton, A. G., Logic for Mathematicians, Cambridge University Press, Cambridge, revised edition (1988) [QA9.H298 1988].
    4. [Heyting] Heyting, A., Intuitionism: An introduction, North-Holland publishing company, Amsterdam, second edition (1966).
    5. [Hitchcock] Hitchcock, David, The peculiarities of Stoic propositional logic, McMaster University; available at (retrieved 3 Jul 2016).
    6. [KalishMontague] Kalish, D. and R. Montague, "On Tarski's formalization of predicate logic with identity," Archiv für Mathematische Logik und Grundlagenforschung, 7:81-101 (1965) [QA.A673].
    7. [Kunen] Kunen, Kenneth, Set Theory: An Introduction to Independence Proofs, Elsevier Science B.V., Amsterdam (1980) [QA248.K75].
    8. [Lopez-Astorga] Lopez-Astorga, Miguel, "The First Rule of Stoic Logic and its Relationship with the Indemonstrables", Revista de Filosofía Tópicos (2016); available at (retrieved 3 Jul 2016).
    9. [Margaris] Margaris, Angelo, First Order Mathematical Logic, Blaisdell Publishing Company, Waltham, Massachusetts (1967) [QA9.M327].
    10. [Megill] Megill, N., "A Finitely Axiomatized Formalization of Predicate Calculus with Equality," Notre Dame Journal of Formal Logic, 36:435-453 (1995) [QA.N914]; available at (accessed 11 Nov 2014); the PDF preprint has the same content (with corrections) but pages are numbered 1-22, and the database references use the numbers printed on the page itself, not the PDF page numbers.
    11. [Mendelson] Mendelson, Elliott, Introduction to Mathematical Logic, 2nd ed., D. Van Nostrand (1979) [QA9.M537].
    12. [Monk2] Monk, J. Donald, "Substitutionless Predicate Logic with Identity," Archiv für Mathematische Logik und Grundlagenforschung, 7:103-121 (1965) [QA.A673].
    13. [Quine] Quine, Willard van Orman, Set Theory and Its Logic, Harvard University Press, Cambridge, Massachusetts, revised edition (1969) [QA248.Q7 1969].
    14. [Sanford] Sanford, David H., If P, then Q: Conditionals and the Foundations of Reasoning, 2nd ed., Routledge Taylor & Francis Group (2003); ISBN 0-415-28369-8; available at (retrieved 3 Jul 2016).
    15. [Stoll] Stoll, Robert R., Set Theory and Logic, Dover Publications, Inc. (1979) [QA248.S7985 1979].
    16. [TakeutiZaring] Takeuti, Gaisi, and Wilson M. Zaring, Introduction to Axiomatic Set Theory, Springer-Verlag, New York, second edition (1982) [QA248.T136 1982].
    17. [Tarski] Tarski, Alfred, "A Simplified Formalization of Predicate Logic with Identity," Archiv für Mathematische Logik und Grundlagenforschung, 7:61-79 (1965) [QA.A673].
    18. [WhiteheadRussell] Whitehead, Alfred North, and Bertrand Russell, Principia Mathematica to *56, Cambridge University Press, Cambridge, 1962 [QA9.W592 1962].

      This page was last updated on 22-Jul-2015.
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