Non Sequitur (logic) - Affirming The Consequent

Affirming The Consequent

Any argument that takes the following form is a non sequitur

  1. If A is true, then B is true.
  2. B is true.
  3. Therefore, A is true.

Even if the premises and conclusion are all true, the conclusion is not a necessary consequence of the premises. This sort of non sequitur is also called affirming the consequent.

An example of affirming the consequent would be:

  1. If Jackson is a human (A) then Jackson is a mammal. (B)
  2. Jackson is a mammal. (B)
  3. Therefore, Jackson is a human. (A)

While the conclusion may be true, it does not follow from the premises: 'Jackson' could be another type of mammal without also being a human. The truth of the conclusion is independent of the truth of its premises - it is a 'non sequitur'.

Affirming the consequent is essentially the same as the fallacy of the undistributed middle, but using propositions rather than set membership.

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