Affirming the Consequent

This fallacy takes the following form:

P1. If A then B

P2. B

C. Therefore A

Explanation: this fallacy involves reasoning that since one thing implies a second thing, then the presence of the second thing allows us to infer the presence of the first. This is fallacious reasoning, as it confuses necessary and sufficient conditions. Statements of the form ‘if A then B’ mean that A is sufficient for B, so that if A occurs or exists, then so will B. Such statements do not, however, mean that A is necessary for B, which (if true) would entail that the only way for B to be true is for A also to be true. Counterexamples to such reasoning are very easy to provide. For example, if my alarm clock fails to go off them I will be late for work; however the fact that I am late for work does not imply that my alarm clock must have failed to go off, since I might be late for some other reason, for example if there was a traffic accident. My alarm clock failing to go off is thus a sufficient condition for my being late, but it is not necessary.

Example: “If the government enacts sound economic policies, then the economy will perform well. The economy has performed excellently since the last election, therefore the government’s economic policies must have been sound.”

Further Reading

Affirming the consequent: overview from Fallacy Files

Affirming the consequent: overview from Common Sense Atheism

Affirming the consequent: overview from Logical Fallacies

Affirming the consequent: overview from

Affirming the consequent: overview from LogicallyFallacious