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Non-classical logic

Non-classical logics are formal systems that differ in a significant way from standard logical systems such as propositional and predicate logic. There are several ways in which this is commonly the case, including by way of extensions, deviations, and variations. The aim of these departures is to make it possible to construct different models of logical consequence and logical truth.

Examples of non-classical logics
There are many kinds of non-classical logic, which include: • Computability logic is a semantically constructed formal theory of computability—as opposed to classical logic, which is a formal theory of truth—that integrates and extends classical, linear and intuitionistic logics. • Dialectical logic is the system of laws of thought, developed within the Hegelian and Marxist traditions, which seeks to supplement or replace the laws of formal logic. The precise nature of the relation between dialectical and formal logic was hotly debated within the Soviet Union and China. • Dynamic semantics interprets formulas as update functions, opening the door to a variety of nonclassical behaviours • Many-valued logic rejects bivalence, allowing for truth values other than true and false. The most popular forms are three-valued logic, as initially developed by Jan Łukasiewicz, and infinitely-valued logics such as fuzzy logic, which permit any real number between 0 and 1 as a truth value. • Intuitionistic logic rejects the law of the excluded middle, double negation elimination, and part of De Morgan's laws; • Linear logic rejects idempotency of entailment as well; • Paraconsistent logic (e.g., relevance logic) rejects the principle of explosion, and has a close relation to dialetheism; • Quantum logicRelevance logic, linear logic, and non-monotonic logic reject monotonicity of entailment; • Non-reflexive logic (also known as "Schrödinger logics") rejects or restricts the law of identity; == Classification of non-classical logics according to specific authors ==
Classification of non-classical logics according to specific authors
In Deviant Logic (1974) Susan Haack divided non-classical logics into deviant, quasi-deviant, and extended logics. The proposed classification is non-exclusive; a logic may be both a deviation and an extension of classical logic. A few other authors have adopted the main distinction between deviation and extension in non-classical logics. John P. Burgess uses a similar classification but calls the two main classes anti-classical and extra-classical. Although some systems of classification for non-classical logic have been proposed, such as those of Haack and Burgess as described above for example, many people who study non-classical logic ignore these classification systems. As such, none of the classification systems in this section should be treated as standard. In an extension, new and different logical constants are added, for instance the "\Box" in modal logic, which stands for "necessarily". The theory of abstract algebraic logic has also provided means to classify logics, with most results having been obtained for propositional logics. The current algebraic hierarchy of propositional logics has five levels, defined in terms of properties of their Leibniz operator: protoalgebraic, (finitely) equivalential, and (finitely) algebraizable. == See also ==
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