Affirming the Consequent

Disciplines > Argument > Fallacies > Affirming the Consequent

Description | Discussion | Example | See also

Description

If A is true then B is true. B is true. Therefore A is true.

If B follows A, then you can assume you can go back the other way also.

Example

I am in London, England. I am in England, therefore I am in London.

If you are cheating on me, you will be out of the house a lot. You are out of the house a lot, so you must be cheating on me.

Discussion

This assumes that an if...then... statement is commutative, that given 'If A then B', you can also reverse it to 'If B then A'. The B, or 'then' part of the statement is called the 'consequent' (the A is the antecedent).

Affirming the Consequent is one of Aristotle's 13 fallacies.

Classification

Non-sequitur, Syllogistic