 Classical logic

Classical logic identifies a class of formal logics that have been most intensively studied and most widely used. The class is sometimes called standard logic as well.^{[1]}^{[2]} They are characterised by a number of properties:^{[3]}
 Law of the excluded middle and Double negative elimination;
 Law of noncontradiction, and the principle of explosion;
 Monotonicity of entailment and Idempotency of entailment;
 Commutativity of conjunction;
 De Morgan duality: every logical operator is dual to another;
While not entailed by the preceding conditions, contemporary discussions of classical logic normally only include propositional and firstorder logics.^{[4]}^{[5]}
The intended semantics of classical logic is bivalent. With the advent of algebraic logic it became apparent however that classical propositional calculus admits other semantics. In Booleanvalued semantics (for classical propositional logic), the truth values are the elements of an arbitrary Boolean algebra; "true" corresponds to the maximal element of the algebra, and "false" corresponds to the minimal element. Intermediate elements of the algebra correspond to truth values other than "true" and "false". The principle of bivalence holds only when the Boolean algebra is taken to be the twoelement algebra, which has no intermediate elements.
Contents
Examples of classical logics
 Aristotle's Organon introduces his theory of syllogisms, which is a logic with a restricted form of judgments: assertions take one of four forms, All Ps are Q, Some Ps are Q, No Ps are Q, and Some Ps are not Q. These judgments find themselves if two pairs of two dual operators, and each operator is the negation of another, relationships that Aristotle summarised with his square of oppositions. Aristotle explicitly formulated the law of the excluded middle and law of noncontradiction in justifying his system, although these laws cannot be expressed as judgments within the syllogistic framework.
 George Boole's algebraic reformulation of logic, his system of Boolean logic;
 The firstorder logic found in Gottlob Frege's Begriffsschrift.
Nonclassical logics
Main article: Nonclassical logic 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.
 Manyvalued logic, including fuzzy logic, which rejects the law of the excluded middle and allows as a truth value any real number between 0 and 1.
 Intuitionistic logic rejects the law of the excluded middle, double negative elimination, and the De Morgan's laws;
 Linear logic rejects idempotency of entailment as well;
 Modal logic extends classical logic with nontruthfunctional ("modal") operators.
 Paraconsistent logic (e.g., dialetheism and relevance logic) rejects the law of noncontradiction;
 Relevance logic, linear logic, and nonmonotonic logic reject monotonicity of entailment;
In Deviant Logic, Fuzzy Logic: Beyond the Formalism, Susan Haack divided nonclassical logics into deviant, quasideviant, and extended logics.^{[5]}
References
 ^ Nicholas Bunnin; Jiyuan Yu (2004). The Blackwell dictionary of Western philosophy. WileyBlackwell. p. 266. ISBN 9781405106795. http://books.google.com/books?id=OskKWI1YA7AC&pg=PA266.
 ^ L. T. F. Gamut (1991). Logic, language, and meaning, Volume 1: Introduction to Logic. University of Chicago Press. pp. 156–157. ISBN 9780226280851. http://books.google.com/books?id=Z0KhywkpolMC&pg=PA156.
 ^ Gabbay, Dov, (1994). 'Classical vs nonclassical logic'. In D.M. Gabbay, C.J. Hogger, and J.A. Robinson, (Eds), Handbook of Logic in Artificial Intelligence and Logic Programming, volume 2, chapter 2.6. Oxford University Press.
 ^ Shapiro, Stewart (2000). Classical Logic. In Stanford Encyclopedia of Philosophy [Web]. Stanford: The Metaphysics Research Lab. Retrieved October 28, 2006, from http://plato.stanford.edu/entries/logicclassical/
 ^ ^{a} ^{b} Haack, Susan, (1996). Deviant Logic, Fuzzy Logic: Beyond the Formalism. Chicago: The University of Chicago Press.
Further reading
 Graham Priest, An Introduction to NonClassical Logic: From If to Is, 2nd Edition, CUP, 2008, ISBN 9780521670265
 Warren Goldfard, "Deductive Logic", 1st edition, 2003, ISBN 0872206602
Categories: Traditional logic
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