fallacy

Affirming the consequent

Also called: Converse error

Affirming the consequent reasons from "if P then Q" and "Q" to "P". It is a formal fallacy: the conclusion does not follow even if the premises are true.

How it works

Q can be true for reasons other than P. The argument mistakes one possible explanation for the only one.

Examples

"If the server is down, the site won't load. The site won't load, so the server is down."

The site could fail to load because of the network, the browser or a bad update.

"If he's guilty, he'd be nervous. He's nervous, so he's guilty."

Innocent people can be nervous for many reasons.

How to spot it

"If P then Q" followed by "Q, therefore P".
One explanation chosen without ruling out others.
Symptoms treated as proof of a single cause.

How to respond

List other ways Q could be true.
Ask what evidence points to P specifically.
Check the logic with a counterexample that has the same form.