Fallacy · Slippery words and shapes
Affirming the consequent
Also called: “If A then B; B, so A”
Reasoning from “if A then B” and “B” to “A”, which does not follow.
What it is
“If it rained, the street is wet” is true. But a wet street does not prove it rained: a street cleaner may have passed. The shape looks like valid reasoning, which is why it fools us, but B can have many causes besides A.
An example
“If he were guilty, he’d be nervous in the interview. He was nervous, so he’s guilty.” Innocent people get nervous in interviews too.
How to spot it
The argument runs backwards along an “if… then…”: from the result to one particular cause.
How to answer it
“What else could explain B, apart from A?”
When it is not a fallacy
Reasoning from effect to likely cause is useful as a hypothesis (it is how detectives start), as long as other causes are weighed rather than ruled out by the shape of the argument.
Printable sheet of all fallacies
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A plain reference you can paste into notes or an essay:
Quest2Reason. “Affirming the consequent.” https://quest2reason.com/learn/ideas/fallacies/affirming-the-consequent. Accessed 13 November 2026.