Affirming the Consequent

• Asserting the Consequent
• Affirmation of the Consequent

Exposition:

Affirming the Consequent is a non-validating form of argument in propositional logic; for instance, let "p" be false and "q" be true, then there is no inconsistency in supposing that the first, conditional premiss is true, which makes the premisses true and the conclusion false.

Together with its similar sibling fallacy, Denying the Antecedent, instances of Affirming the Consequent are most likely to seem valid when we assume the converse of the argument's conditional premiss. In the Example, for instance, we may assume:

(Suppressed Premiss) If the streets are wet then it's raining.

Since wet streets usually dry rapidly, it is a good rule of thumb that wet streets indicate rain. With this suppressed premiss, the argument in the Example is valid. So, in general, in an instance of the form Affirming the Consequent, if it is reasonable to consider the converse of the conditional premiss to be a suppressed premiss, then the argument is not fallacious, but a valid enthymeme.

In contrast, it would not be reasonable to consider the Counter-Example to be an enthymeme, since the converse of its conditional premiss is not plausible, namely:

If the streets are covered with snow then it's snowing.

Unlike rain, we know, at cold temperatures it takes snow a very long time to evaporate. So that, while snow on the ground is a good sign of past snowing, it's a bad sign of present snowing. Thus, the Counter-Example is a fallacious instance of Affirming the Consequent.

Sibling Fallacy: Denying the Antecedent

Source:

A. R. Lacey, A Dictionary of Philosophy (Third Revised Edition) (Barnes & Noble, 1996).