Multivalued logic
From Academic Kids

Multivalued logics are logical calculi in which there are more than two possible truth values. Traditionally, logical calculi are bivalent—that is, there are only two possible truth values for any proposition, true and false (which generally correspond to our intuitive notions of truth and falsity). But bivalence is only one possible range of truth values that may be assigned, and other logical systems have been developed with variations on bivalence, or with more than two possible truthvalue assignments.
In the classical bivalence scheme, true and false are determinate values: a proposition is either true or false (exclusively), and if the proposition does not have one of those values, by definition it must have the other. This is the justification for the law of excluded middle: P ∨ ¬P—i.e., the proposition or its negation holds.
One point to remember is that logic is a system for preserving some property of propositions across transformations. In classical logic, this property is "truth": In a valid argument, the truth of the derived proposition is guaranteed because the application of valid steps preserves the property. However, that property doesn't have to be that of "truth"; instead, it can be some other concept.
For example, the preserved property could be justification, the foundational concept of intuitionistic logic. Thus, a proposition is not true or false; instead, it is justified or not. A key difference between justification and truth, in this case, is that the law of the excluded middle doesn't hold: a proposition that is not not justified is not necessarily justified; instead, it's only not proven that it's not justified. The key difference is the determinacy of the preserved property: One may prove that P is justified, that P is not justified, or be unable to prove either. A valid argument preserves justification across transformations, so a proposition derived from justified propositions is still justified. However, there are proofs in classical logic that depend upon the law of excluded middle; since that law is not usable under this scheme, there are propositions that cannot be proven that way.
Fuzzy logics were introduced by Lotfi Zadeh as a formalization of vagueness; i.e., the phenomenon that a predicate may apply to an object not absolutely, but to a certain degree, and that there may be borderline cases. Such logics can be used to deal with the sorites paradox. Instead of the two truth values "true" and "false", fuzzy logic employs infinitely many values between 0, corresponding to "absolutely false", and 1 corresponding to "absolutely true". A borderline case might then be assigned a truth value of 0.5. One can apply these systems of logic as the foundation of fuzzy set theories.
Another example of an infinitelyvalued logic is probability logic.
History
The first known logician who didn't fully accept the law of the excluded middle was Aristotle (De Interpretatione, ch. IX), though he didn't create a system of multivalued logic. The law of excluded middle was accepted by stoic philosophers (the law may originate from one of them, Chrysippus). The later logicians until the coming of the 20th followed Aristotelian logic, except regarding the excluded middle.
The 20th century brought the idea of multivalued logic back. The Polish logician and philosopher Jan Łukasiewicz began to create systems of manyvalued logic in 1920, using a third value foralso "possible" to deal with Aristotle's paradox of the sea battle. Meanwhile, the American mathematician Emil L. Post (1921) also introduced the formulation of additional truth degrees. Gödel in 1932 showed that intuitionistic logic is not a finitelymany valued logic, and defined a system of Gödel logics intermediate between classical and intuitionistic logic; such logics are known as intermediate logics.
External links
 Stanford Encyclopedia of Philosophy entry (http://plato.stanford.edu/entries/logicmanyvalued/)