Affirming the Consequent

When Bob drinks coffee after 2 pm, he sleeps poorly. He slept poorly, so he must have had coffee after 2 pm.

Affirming the consequent takes this form:
If P, then Q.
Q.
Therefore, P.

In the coffee example,
P: Bob drinks coffee after 2 pm.
Q: Bob doesn’t sleep well.

It is a fallacy because P is not necessarily the only thing that causes Q. For example, alcohol might cause Bob to sleep poorly, too.

This can be tricky because the fallacy looks similar to modus ponens, a valid argument form:

If P, then Q.
P.
Therefore, Q.

Example:
If Bob drinks coffee after 2 pm, he doesn’t sleep well. Bob drank coffee after 2 pm, so he didn’t sleep well. (valid)

Back to the Formal Fallacy Handbook

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