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In classical logic, a contradiction consists of a logical incompatibility between two or more propositions. It occurs when the propositions, taken together, yield two conclusions which form the logical, usually opposite inversions of each other. Illustrating a general tendency in applied logic, Aristotle's law of noncontradiction states that "One cannot say of something that it is and that it is not in the same respect and at the same time."
By extension, outside of classical logic, one can speak of contradictions between actions when one presumes that their motives contradict each other.
By creation of a paradox, Plato's Euthydemus dialogue demonstrates the need for the notion of contradiction. In the ensuing dialogue Dionysodorus denies the existence of "contradiction", all the while that Socrates is contradicting him:
Indeed, Dionysodorus agrees that "there is no such thing as false opinion . . . there is no such thing as ignorance" and demands of Socrates to "Refute me." Socrates responds "But how can I refute you, if, as you say, to tell a falsehood is impossible?".^{[1]}
In classical logic, particularly in propositional and firstorder logic, a proposition is a contradiction if and only if . Since for contradictory it is true that for all (because ), one may prove any proposition from a set of axioms which contains contradictions. This is called the "principle of explosion" or "ex falso quodlibet" ("from falsity, whatever you like").
In a complete logic, a formula is contradictory if and only if it is unsatisfiable.
For a proposition it is true that , i. e. that is a tautology, i. e. that it is always true, if and only if , i. e. if the negation of is a contradiction. Therefore, a proof that also proves that is true. The use of this fact constitutes the technique of the proof by contradiction, which mathematicians use extensively. This applies only in a logic using the excluded middle as an axiom.
In mathematics, the symbol used to represent a contradiction within a proof varies. [1] Some symbols that may be used to represent a contradiction include ↯, Opq, , ⊥, ↮, and ※. It is not uncommon to see Q.E.D. or some variant immediately after a contradiction symbol; this occurs in a proof by contradiction, to indicate that the original assumption was false and that the theorem must therefore be true.
A Consistency proof requires (i) an axiomatic system (ii) a demonstration that it is not the case that both the formula p and its negation ~p can derived in the system. But by whatever method one goes about it, all consistency proofs would seem to necessitate the primitive notion of contradiction; moreover, it seems as if this notion would simultaneously have to be "outside" the formal system in the definition of tautology.
When Emil Post in his 1921 Introduction to a general theory of elementary propositions extended his proof of the consistency of the propositional calculus (i.e. the logic) beyond that of Principia Mathematica (PM) he observed that with respect to a generalized set of postulates (i.e. axioms) he would no longer be able to automatically invoke the notion of "contradiction" – such a notion might not be contained in the postulates:
Post's solution to the problem is described in the demonstration An Example of a Successful Absolute Proof of Consistency offered by Ernest Nagel and James R. Newman in their 1958 Gödel's Proof. They too observe a problem with respect to the notion of "contradiction" with its usual "truth values" of "truth" and "falsity". They observe that:
Given some "primitive formulas" such as PM's primitives S_{1} V S_{2} [inclusive OR], ~S (negation) one is forced to define the axioms in terms of these primitive notions. In a thorough manner Post demonstrates in PM, and defines (as do Nagel and Newman, see below), that the property of tautologous – as yet to be defined – is "inherited": if one begins with a set of tautologous axioms (postulates) and a deduction system that contains substitution and modus ponens then a consistent system will yield only tautologous formulas.
So what will be the definition of tautologous?
Nagel and Newman create two mutually exclusive and exhaustive classes K_{1} and K_{2} into which fall (the outcome of) the axioms when their variables e.g. S_{1} and S_{2} are assigned from these classes. This also applies to the primitive formulas. For example: "A formula having the form S_{1} V S_{2} is placed into class K_{2} if both S_{1} and S_{2} are in K_{2}; otherwise it is placed in K_{1}", and "A formula having the form ~S is placed in K_{2}, if S is in K_{1}; otherwise it is placed in K_{1}".^{[4]}
Nagel and Newman can now define the notion of tautologous: "a formula is a tautology if, and only if, it falls in the class K_{1} no matter in which of the two classes its elements are placed".^{[5]} Now the property of "being tautologous" is described without reference to a model or an interpretation.
Post observed that, if the system were inconsistent, a deduction in it (that is, the last formula in a sequence of formulas derived from the tautologies) could ultimately yield S itself. As an assignment to variable S can come from either class K_{1} or K_{2}, the deduction violates the inheritance characteristic of tautology, i.e. the derivation must yield an (evaluation of a formula) that will fall into class K_{1}. From this, Post was able to derive the following definition of inconsistency without the use of the notion of contradiction:
In other words, the notion of "contradiction" can be dispensed when constructing a proof of consistency; what replaces it is the notion of "mutually exclusive and exhaustive" classes. More interestingly,^{[citation needed]} an axiomatic system need not include the notion of "contradiction".
Adherents of the epistemological theory of coherentism typically claim that as a necessary condition of the justification of a belief, that belief must form a part of a logically noncontradictory (consistent) system of beliefs. Some dialetheists, including Graham Priest, have argued that coherence may not require consistency.^{[6]}
A pragmatic contradiction occurs when the very statement of the argument contradicts the claims it purports. An inconsistency arises, in this case, because the act of utterance, rather than the content of what is said, undermines its conclusion.^{[7]} For examples, arguably, Nietzsche's statement that one should not obey others, or Moore's paradox. Within the analytic tradition, these are seen as selfrefuting statements and performative contradictions. Other traditions may read them more like zen koans, in which the author purposes makes a contradiction using the traditional meaning, but then implies a new meaning of the word which does not contradict the statement.
In dialectical materialism, contradiction, as derived by Karl Marx from Hegelianism, usually refers to an opposition inherently existing within one realm, one unified force or object. This contradiction, as opposed to metaphysical thinking, is not an objectively impossible thing, because these contradicting forces exist in objective reality, not cancelling each other out, but actually defining each other's existence. According to Marxist theory, such a contradiction can be found, for example, in the fact that:
Hegelian and Marxist theory stipulates that the dialectic nature of history will lead to the sublation, or synthesis, of its contradictions. Marx therefore postulated that history would logically make capitalism evolve into a socialist society where the means of production would equally serve the exploited and suffering class of society, thus resolving the prior contradiction between (a) and (b).^{[citation needed]}
Mao Zedong's philosophical essay furthered Marx and Lenin's thesis and suggested that all existence is the result of contradiction.^{[8]}
Colloquial usage can label actions and/or statements as contradicting each other when due (or perceived as due) to presuppositions which are contradictory in the logical sense.
Proof by contradiction is used in mathematics to construct proofs.
Look up contradiction or although in Wiktionary, the free dictionary. 
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リンク元  「矛盾」「conflict」「incompatibility」「discrepancy」 
拡張検索  「contradictory」「contradiction」 