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Modus ponendo tollens

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

  1. <math> \neg (A \and B)</math>
  2. <math> A\,\! </math>
  3. <math> \therefore \neg B</math>

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

  1. <math> A\,|\,B</math>
  2. <math> A\,\! </math>
  3. <math> \therefore \neg B</math>


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