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| {{About|the ex falso quodlibet logical principle|the file tagging software|Ex Falso (software)}}
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| {{One source|date= December 2011}}
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| The '''principle of explosion''', ([[Latin]]: ''ex falso quodlibet'' or ''ex contradictione sequitur quodlibet'', "from a contradiction, anything follows") or the '''principle of Pseudo-Scotus''', is the law of [[classical logic]], [[intuitionistic logic]] and similar logical systems, according to which any statement can be proven from a contradiction.<ref>Carnielli, W. and Marcos, J. (2001) [http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.107.70 "Ex contradictione non sequitur quodlibet"] ''Proc. 2nd Conf. on Reasoning and Logic'' (Bucharest, July 2000)</ref> That is, once a contradiction has been asserted, any proposition (or its negation) can be inferred from it.
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| As a demonstration of the principle, consider two contradictory statements - “All lemons are yellow” and "Not all lemons are yellow", and suppose (for the sake of argument) that both are simultaneously true. If that is the case, anything can be proven, e.g. "Santa Claus exists", by using the following argument:
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| # We know that "All lemons are yellow" as it is defined to be true.
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| # Therefore the statement that (“All lemons are yellow" OR "Santa Claus exists”) must also be true, since the first part is true.
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| # However, if "Not all lemons are yellow" (and this is also defined to be true), Santa Claus must exist - otherwise statement 2 would be false. It has thus been "proven" that Santa Claus exists. The same could be applied to any assertion, including the statement "Santa Claus does not exist".
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| ==Symbolic representation==
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| The principle of explosion can be expressed in the following way (where "<math>\vdash</math>" symbolizes the relation of [[logical consequence]]):
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| : <math>\{ \phi , \lnot \phi \} \vdash \psi</math>
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| : ''or''
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| : <math>\bot \to P</math>.
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| This can be read as, "If one claims something is both true (<math>\phi\,</math>) and not true (<math>\lnot \phi</math>), one can logically derive ''any'' conclusion (<math>\psi</math>)."
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| ==Arguments for explosion==
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| An informal, descriptive, argument is given above. In more formal terms, there are two kinds of argument for the principle of explosion, semantic and proof-theoretic.
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| ===The semantic argument===
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| The first argument is ''semantic'' or ''[[model theory|model-theoretic]]'' in nature. A sentence <math>\psi</math> is a ''[[semantic consequence]]'' of a set of sentences <math>\Gamma</math> only if every model of <math>\Gamma</math> is a model of <math>\psi</math>. But there is no model of the contradictory set <math>\{\phi , \lnot \phi \}</math>. [[A fortiori]], there is no model of <math>\{\phi , \lnot \phi \}</math> that is not a model of <math>\psi</math>. Thus, vacuously, every model of <math>\{\phi , \lnot \phi \}</math> is a model of <math>\psi</math>. Thus <math>\psi</math> is a semantic consequence of <math>\{\phi , \lnot \phi \}</math>.
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| === The proof-theoretic argument ===
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| The second type of argument is ''[[proof theory|proof-theoretic]]'' in nature. Consider the following derivations:
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| #<math>\phi \wedge \neg \phi\,</math>
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| #:assumption
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| #<math>\phi\,</math>
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| #:from (1) by [[conjunction elimination]]
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| #<math>\neg \phi\,</math>
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| #:from (1) by conjunction elimination
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| #<math>\phi \vee \psi\,</math>
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| #:from (2) by [[disjunction introduction]]
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| #<math>\psi\,</math>
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| #:from (3) and (4) by [[disjunctive syllogism]]
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| #<math>(\phi \wedge \neg \phi) \to \psi</math>
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| #:from (5) by [[conditional proof]] (discharging assumption 1)
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| This is just the symbolic version of the informal argument given above, with <math>\phi</math> standing for "all lemons are yellow" and <math>\psi</math> standing for "Santa Claus exists". From "all lemons are yellow and not all lemons are yellow" (1), we infer "all lemons are yellow" (2) and "not all lemons are yellow" (3); from "all lemons are yellow" (2), we infer "all lemons are yellow or Santa Claus exists" (4); and from "not all lemons are yellow" (3) and "all lemons are yellow or Santa Claus exists" (4), we infer "Santa Claus exists" (5). Hence, if all lemons are yellow and not all lemons are yellow, then Santa Claus exists.
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| Or:
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| #<math>\phi \wedge \neg \phi\,</math>
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| #:hypothesis
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| #<math>\phi\,</math>
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| #:from (1) by conjunction elimination
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| #<math>\neg \phi\,</math>
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| #:from (1) by conjunction elimination
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| #<math>\neg \psi\,</math>
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| #:hypothesis
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| #<math>\phi\,</math>
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| #:reiteration of (2)
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| #<math>\neg \psi \to \phi</math>
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| #:from (4) to (5) by [[deduction theorem]]
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| #<math>( \neg \phi \to \neg \neg \psi)</math>
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| #:from (6) by [[contraposition]]
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| #<math>\neg \neg \psi</math>
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| #:from (3) and (7) by [[modus ponens]]
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| #<math>\psi\,</math>
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| #:from (8) by [[double negation elimination]]
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| #<math>(\phi \wedge \neg \phi) \to \psi</math>
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| #:from (1) to (9) by deduction theorem
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| Or:
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| #<math>\phi \wedge \neg \phi\,</math>
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| #:assumption
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| #<math>\neg \psi\,</math>
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| #:assumption
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| #<math>\phi\,</math>
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| #:from (1) by conjunction elimination
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| #<math>\neg \phi\,</math>
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| #:from (1) by conjunction elimination
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| #<math>\neg \neg \psi\,</math>
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| #:from (3) and (4) by [[reductio ad absurdum]] (discharging assumption 2)
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| #<math>\psi\,</math>
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| #:from (5) by double negation elimination
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| #<math>(\phi \wedge \neg \phi) \to \psi</math>
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| #:from (6) by conditional proof (discharging assumption 1)
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| == Addressing the principle ==
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| [[Paraconsistent logic]]s have been developed that allow for sub-contrary forming operators. [[Formal semantics (logic)|Model-theoretic]] paraconsistent logicians often deny the assumption that there can be no model of <math>\{\phi , \lnot \phi \}</math> and devise semantical systems in which there are such models. Alternatively, they reject the idea that propositions can be classified as true or false. [[Proof-theoretic semantics|Proof-theoretic]] paraconsistent logics usually deny the validity of one of the steps necessary for deriving an explosion, typically including disjunctive syllogism, disjunction introduction, and [[reductio ad absurdum]].
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| ==Use==
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| The [[metamathematics|metamathematical]] value of the principle of explosion is that for any logical system where this principle holds, any derived [[mathematical theory|theory]] which proves [[false (logic)|<math>\bot</math>]] (or an equivalent form, <math>\phi \land \lnot \phi</math>) is worthless because ''all'' its [[statement (logic)|statements]] would become [[theorem]]s, making it impossible to distinguish [[truth]] from falsehood. That is to say, the principle of explosion is an argument for the [[law of non-contradiction]] in classical logic, because without it all truth statements become meaningless.
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| ==See also==
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| * [[Consequentia mirabilis]] - Clavius's Law
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| * [[Dialetheism]] – belief in the existence of true contradictions
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| * [[Law of excluded middle]] – every proposition is either true or not true
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| * [[Law of noncontradiction]] – no proposition can be both true and not true
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| * [[Paraconsistent logic]] – a [[modal logic]] used to address contradictions
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| * [[Paradox of entailment]] – a seeming paradox derived from the principle of explosion
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| * [[Reductio ad absurdum]] – concluding that a proposition is false because it produces a contradiction
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| * [[Trivialism]] – the belief that all statements of the form "P and not-P" are true
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| ==References==
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| {{reflist}}
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| [[Category:Theorems in propositional logic]]
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| [[Category:Classical logic]]
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| [[Category:Principles]]
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