**Non-classical logics** (and sometimes **alternative 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 done, 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.^{[1]}

"Philosophical logic is understood to encompass and focus on non-classical logics, although the term has other meanings as well.^{[2]} In addition, some parts of "theoretical computer science can be thought of as using non-classical reasoning, although this varies according to the subject area. For example, the basic "boolean functions (e.g. "AND, "OR, "NOT, etc) in "computer science are very much "classical in nature, as is clearly the case given the fact that they can be fully described by classical "truth tables. However, in contrast, some "computerized proof methods may not use classical logic in the reasoning process.

There are many kinds of non-classical logic, which include:

- "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.
- "Intuitionistic logic rejects the "law of the excluded middle, "double negation elimination, and "De Morgan's laws;
- "Linear logic rejects "idempotency of "entailment as well;
- "Modal logic extends classical logic with "non-truth-functional ("modal") operators.
- "Paraconsistent logic (e.g., "relevance logic) rejects the "principle of explosion, and closely related to "dialetheism;
- "Relevance 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;
^{[3]} - "Computability logic is a semantically constructed formal theory of computability, as opposed to classical logic, which is a formal theory of truth; integrates and extends classical, linear and intuitionistic logics.

In *Deviant Logic* (1974) "Susan Haack divided non-classical logics into "deviant, quasi-deviant, and extended logics.^{[4]} The proposed classification is non-exclusive; a logic may be both a deviation and an extension of classical logic.^{[5]} A few other authors have adopted the main distinction between deviation and extension in non-classical logics.^{[6]}^{[7]}^{[8]} "John P. Burgess uses a similar classification but calls the two main classes anti-classical and extra-classical.^{[9]} 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 "" in "modal logic, which stands for "necessarily."^{[6]} In extensions of a logic,

- the set of "well-formed formulas generated is a "proper superset of the set of well-formed formulas generated by "classical logic.
- the set of "theorems generated is a proper superset of the set of theorems generated by classical logic, but only in that the novel theorems generated by the extended logic are only a result of novel well-formed formulas.

(See also "Conservative extension.)

In a *deviation*, the usual logical constants are used, but are given a different meaning than usual. Only a subset of the theorems from the classical logic hold. A typical example is intuitionistic logic, where the "law of excluded middle does not hold.^{[8]}^{[9]}

Additionally, one can identify a *variations* (or *variants*), where the content of the system remains the same, while the notation may change substantially. For instance "many-sorted predicate logic is considered a just variation of predicate logic.^{[6]}

This classification ignores however semantic equivalences. For instance, "Gödel showed that all theorems from intuitionistic logic have an equivalent theorem in the classical modal logic S4. The result has been generalized to "superintuitionistic logics and extensions of S4.^{[10]}

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.^{[11]}

**^***Logic for philosophy*, "Theodore Sider**^**"John P. Burgess (2009).*Philosophical logic*. Princeton University Press. pp. vii–viii. "ISBN "978-0-691-13789-6.**^**da Costa, Newton (1994),*Schrödinger logics*, Studia Logica, p. 533.**^**"Haack, Susan (1974).*Deviant logic: some philosophical issues*. CUP Archive. p. 4. "ISBN "978-0-521-20500-9.**^**"Haack, Susan (1978).*Philosophy of logics*. Cambridge University Press. p. 204. "ISBN "978-0-521-29329-7.- ^
^{a}^{b}^{c}"L. T. F. Gamut (1991).*Logic, language, and meaning, Volume 1: Introduction to Logic*. University of Chicago Press. pp. 156–157. "ISBN "978-0-226-28085-1. **^**Seiki Akama (1997).*Logic, language, and computation*. Springer. p. 3. "ISBN "978-0-7923-4376-9.- ^
^{a}^{b}Robert Hanna (2006).*Rationality and logic*. MIT Press. pp. 40–41. "ISBN "978-0-262-08349-2. - ^
^{a}^{b}John P. Burgess (2009).*Philosophical logic*. Princeton University Press. pp. 1–2. "ISBN "978-0-691-13789-6. **^**Dov M. Gabbay; Larisa Maksimova (2005).*Interpolation and definability: modal and intuitionistic logics*. Clarendon Press. p. 61. "ISBN "978-0-19-851174-8.**^**D. Pigozzi (2001). "Abstract algebraic logic". In M. Hazewinkel.*Encyclopaedia of mathematics: Supplement Volume III*. Springer. pp. 2–13. "ISBN "1-4020-0198-3. Also online: "Hazewinkel, Michiel, ed. (2001) [1994], "Abstract algebraic logic",*"Encyclopedia of Mathematics*, Springer Science+Business Media B.V. / Kluwer Academic Publishers, "ISBN "978-1-55608-010-4

- "Graham Priest (2008).
*An introduction to non-classical logic: from if to is*(2nd ed.). Cambridge University Press. "ISBN "978-0-521-85433-7. - "Dov M. Gabbay (1998).
*Elementary logics: a procedural perspective*. Prentice Hall Europe. "ISBN "978-0-13-726365-3. A revised version was published as D. M. Gabbay (2007).*Logic for Artificial Intelligence and Information Technology*. "College Publications. "ISBN "978-1-904987-39-0. - "John P. Burgess (2009).
*Philosophical logic*. "Princeton University Press. "ISBN "978-0-691-13789-6. Brief introduction to non-classical logics, with a primer on the classical one. - Lou Goble, ed. (2001).
*The Blackwell guide to philosophical logic*. Wiley-Blackwell. "ISBN "978-0-631-20693-4. Chapters 7-16 cover the main non-classical logics of broad interest today. - Lloyd Humberstone (2011).
*The Connectives*. MIT Press. "ISBN "978-0-262-01654-4. Probably covers more logics than any of the other titles in this section; a large part of this 1500-page monograph is cross-sectional, comparing—as its title implies—the "logical connectives in various logics; decidability and complexity aspects are generally omitted though.