Principle of explosion

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This article is about the ex falso quodlibet logical principle. For the file tagging software, see Ex Falso (software).

The principle of explosion (Latin: ex falso quodlibet, "from a falsehood, anything follows", 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.[1] That is, once a contradiction has been asserted, any proposition (or its negation) can be inferred from it.

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:

  1. We know that "All lemons are yellow" as it is defined to be true.
  2. Therefore the statement that (“All lemons are yellow" AND/OR "Santa Claus exists”) must also be true, since the first part is true.
  3. 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".

Symbolic representation[edit]

The principle of explosion can be expressed in the following way (where "\vdash" symbolizes the relation of logical consequence and "\bot " symbolizes a contradiction) :

\{ \phi , \lnot \phi \} \vdash \psi
or
\bot \to P.

This can be read as, "If one claims something (\phi\,) and its negation (\lnot \phi), one can logically derive any conclusion (\psi)."

Arguments for explosion[edit]

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.

The semantic argument[edit]

The first argument is semantic or model-theoretic in nature. A sentence \psi is a semantic consequence of a set of sentences \Gamma only if every model of \Gamma is a model of \psi. But there is no model of the contradictory set \{\phi , \lnot \phi \}. A fortiori, there is no model of \{\phi , \lnot \phi \} that is not a model of \psi. Thus, vacuously, every model of \{\phi , \lnot \phi \} is a model of \psi. Thus \psi is a semantic consequence of \{\phi , \lnot \phi \}.

The proof-theoretic argument[edit]

The second type of argument is proof-theoretic in nature. Consider the following derivations:

  1. \phi \wedge \neg \phi\,
    assumption
  2. \phi\,
    from (1) by conjunction elimination
  3. \neg \phi\,
    from (1) by conjunction elimination
  4. \phi \vee \psi\,
    from (2) by disjunction introduction
  5. \psi\,
    from (3) and (4) by disjunctive syllogism
  6. (\phi \wedge \neg \phi) \to \psi
    from (5) by conditional proof (discharging assumption 1)

This is just the symbolic version of the informal argument given above, with \phi standing for "all lemons are yellow" and \psi 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.

Or:

  1. \phi \wedge \neg \phi\,
    hypothesis
  2. \phi\,
    from (1) by conjunction elimination
  3. \neg \phi\,
    from (1) by conjunction elimination
  4. \neg \psi\,
    hypothesis
  5. \phi\,
    reiteration of (2)
  6. \neg \psi \to \phi
    from (4) to (5) by deduction theorem
  7. ( \neg \phi \to \neg \neg \psi)
    from (6) by contraposition
  8. \neg \neg \psi
    from (3) and (7) by modus ponens
  9. \psi\,
    from (8) by double negation elimination
  10. (\phi \wedge \neg \phi) \to \psi
    from (1) to (9) by deduction theorem

Or:

  1. \phi \wedge \neg \phi\,
    assumption
  2. \neg \psi\,
    assumption
  3. \phi\,
    from (1) by conjunction elimination
  4. \neg \phi\,
    from (1) by conjunction elimination
  5. \neg \neg \psi\,
    from (3) and (4) by reductio ad absurdum (discharging assumption 2)
  6. \psi\,
    from (5) by double negation elimination
  7. (\phi \wedge \neg \phi) \to \psi
    from (6) by conditional proof (discharging assumption 1)

Addressing the principle[edit]

Paraconsistent logics have been developed that allow for sub-contrary forming operators. Model-theoretic paraconsistent logicians often deny the assumption that there can be no model of \{\phi , \lnot \phi \} 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 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.

Use[edit]

The metamathematical value of the principle of explosion is that for any logical system where this principle holds, any derived theory which proves \bot (or an equivalent form, \phi \land \lnot \phi) is worthless because all its statements would become theorems, 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.

See also[edit]

References[edit]

  1. ^ Carnielli, W. and Marcos, J. (2001) "Ex contradictione non sequitur quodlibet" Proc. 2nd Conf. on Reasoning and Logic (Bucharest, July 2000)