Minimal logic, or minimal calculus, is a symbolic logic system originally developed by Ingebrigt Johansson. It is an intuitionistic and paraconsistent logic, that rejects both the law of the excluded middle as well as the principle of explosion (ex falso quodlibet), and therefore holding neither of the following two derivations as valid:
where is any proposition. Most constructive logics only reject the former, the law of excluded middle. In classical logic, the ex falso laws
as well as their variants with and switched, are equivalent to each other and valid. Minimal logic also rejects those principles.
Like intuitionistic logic, minimal logic can be formulated in the language using an implication , a conjunction , a disjunction , and falsum or absurdity as the basic connectives. Negation is treated as an abbreviation for . Minimal logic is axiomatized as the positive fragment of intuitionistic logic.
Relation to classical logic
Adding the double negation law to minimal logic brings the calculus back to classical logic.
Relation to intuitionistic logic
The propositional form of modus ponens,
is clearly valid also in minimal logic.
Constructively, represents a proposition for which there can be no reason to believe it. To prove propositions of the form , one shows that assuming leads to a contradiction, . With the principle of explosion this stated as
the principle of explosion expresses that to derive any proposition one may also do this by deriving absurdity . This principle is rejected in minimal logic. This means the formula does not axiomatically hold for arbitrary and .
As minimal logic represents only the positive fragment of intuitionistic logic, it is a subsystem of intuitionistic logic and is strictly weaker.
Practically, this enables the disjunctive syllogism the intuitionistic context:
Given a constructive proof of and constructive rejection of , the principle of explosion unconditionally allows for the positive case choice of . This is because if was proven by proving then is already proven, while if was proven by proving , then also follows if the system allows for explosion.
Note that with taken for in the modus ponens expression, the law of non-contradiction
i.e. , can still be proven in minimal logic. Moreover, any formula using only is provable in minimal logic if and only if it is provable in intuitionistic logic.
Relation to type theory
Use of negation
Absurdity is necessary in natural deduction, as well as type theoretical formulations under the Curry–Howard correspondence. In type systems, is often also introduced as the empty type. In many contexts, need not be as a separate constant in the logic but its role can be replaced with any rejected proposition. For example, it can be defined as where ought to be distinct, such as in a theory involving natural numbers. For example, with the above example, proving , i.e. proving to be false, means to prove .
One may consider the system obtained by restricting minimal logic to implication only. In this setting, it can be defined by the following sequent rules:
There are semantics of minimal logic that mirror the frame-semantics of intuitionistic logic, see the discussion of semantics in paraconsistent logic. Here the valuation functions assigning truth and falsity to propositions can be subject to less constraints.
- Ingebrigt Johansson (1937). "Der Minimalkalkül, ein reduzierter intuitionistischer Formalismus". Compositio Mathematica (in German). 4: 119–136.
- M. Weber and M. Simons and C. Lafontaine (1993). The Generic Development Language DEVA: Presentation and Case Studies. LNCS. 738. Springer. p. 246. Here: p.36-40.
- Gérard Huet (May 1986). Formal Structures for Computation and Deduction. International Summer School on Logic of Programming and Calculi of Discrete Design. Marktoberdorf. Here: p.125, p.132
- A.S. Troelstra and H. Schwichtenberg, 2000, Basic Proof Theory, Cambridge University Press, ISBN 0521779111