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Modus ponendo tollens

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Modus ponendo tollens (MPT;[1] Latin: "mode that denies by affirming")[2] is a valid rule of inference for propositional logic. It is closely related to modus ponens and modus tollendo ponens.

Overview

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MPT is usually described as having the form:

  1. Not both A and B
  2. A
  3. Therefore, not B

For example:

  1. Ann and Bill cannot both win the race.
  2. Ann won the race.
  3. Therefore, Bill cannot have won the race.

As E. J. Lemmon describes it: "Modus ponendo tollens is the principle that, if the negation of a conjunction holds and also one of its conjuncts, then the negation of its other conjunct holds."[3]

In logic notation this can be represented as:

Based on the Sheffer Stroke (alternative denial), "|", the inference can also be formalized in this way:

Proof

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Step Proposition Derivation
1 Given
2 Given
3 De Morgan's laws (1)
4 Double negation (2)
5 Disjunctive syllogism (3,4)

Strong form

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Modus ponendo tollens can be made stronger by using exclusive disjunction instead of non-conjunction as a premise:

See also

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References

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  1. ^ Politzer, Guy & Carles, Laure. 2001. 'Belief Revision and Uncertain Reasoning'. Thinking and Reasoning. 7:217–234.
  2. ^ Stone, Jon R. (1996). Latin for the Illiterati: Exorcizing the Ghosts of a Dead Language. London: Routledge. p. 60. ISBN 0-415-91775-1.
  3. ^ Lemmon, Edward John. 2001. Beginning Logic. Taylor and Francis/CRC Press, p. 61.