Affirming The Consequent
Any argument that takes the following form is a non sequitur
- If A is true, then B is true.
- B is true.
- 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:
- If Jackson is a human (A) then Jackson is a mammal. (B)
- Jackson is a mammal. (B)
- 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.
Read more about this topic: Non Sequitur (logic)
Famous quotes containing the words affirming the, affirming and/or consequent:
“A large part of the popularity and persuasiveness of psychology comes from its being a sublimated spiritualism: a secular, ostensibly scientific way of affirming the primacy of “spirit” over matter.”
—Susan Sontag (b. 1933)
“Most governments have been based, practically, on the denial of equal rights of men ... ours began, by affirming those rights. They said, some men are too ignorant, and vicious, to share in government. Possibly so, said we; and, by your system, you would always keep them ignorant, and vicious. We proposed to give all a chance; and we expected the weak to grow stronger, the ignorant wiser; and all better, and happier together.”
—Abraham Lincoln (1809–1865)
“One of the many to whom, from straightened circumstances, a consequent inability to form the associations they would wish, and a disinclination to mix with the society they could obtain, London is as complete a solitude as the plains of Syria.”
—Charles Dickens (1812–1870)