Steelman
Affirming the consequent
Also called converse error, fallacy of the consequent.
If P then Q; Q is true; so P must be true. Invalid.
What it is
Reasoning from “If P, then Q” and “Q” to the conclusion “P”. It is the invalid look-alike of modus ponens.
The pattern
If P then Q. Q. Therefore, P. (invalid) Valid look-alike (modus ponens): If P then Q. P. Therefore, Q.
An example
“If it rained last night, the lawn would be wet. The lawn is wet. So it rained last night.”
The sprinkler, morning dew or a burst pipe could also wet the lawn. Counterexample with the same form: “If I’m in Paris, I’m in France. I’m in France. So I’m in Paris.” (I could be in Lyon.)
Why it fails
“If P then Q” says P is enough to guarantee Q. It doesn’t say P is the only way to get Q. Other things can make Q true, so learning that Q is true doesn’t tell you which one did.
When it’s not a fallacy
Most lists skip this part. Calling a fair point a fallacy is a mistake too.
It can be fair when
- It’s offered as evidence, not proof. Q can make P more likely, especially when few other things cause Q. Detectives and doctors use this kind of inference to the best explanation, but it’s a hypothesis to test, not a deduction.
- The conditional is really a biconditional (“P if and only if Q”). Then Q does guarantee P.
Try one
A question from the app. The wrong answers are its usual look-alikes.
Dev, the night of the 2022 final: “Messi’s the greatest ever, and that settles it. For years people said you can’t be the greatest of all time without a World Cup, and that was always the knock on him. Well, he’s got one now, so that knock is gone.”
What’s the flaw?
A World Cup was a requirement for the title, not proof of it. Pelé, Maradona and Zidane all won one too, so meeting the requirement can’t settle who’s the greatest.
A fair reply
“Winning it cleared the last objection, but Pelé and Maradona have one too, so it can’t decide it on its own.”
Often confused with
- Confusing sufficient and necessary
Commutation converts “if P then Q” into “if Q then P” as a rule. Affirming the consequent uses that mistaken reversal to conclude P from Q.
- Denying the antecedent
Its twin: denying the antecedent reasons from “not P” to “not Q”. Both treat a one-way conditional as if it ran both ways.
Where the name comes from
The consequent is the “then” part of an if-then, from the Latin for “following.” Affirming it means saying that part is true, then treating that as proof of the “if” part. The converse error is the same slip: the converse is the if-then flipped around, and this move quietly treats the flip as true. Fallacy of the consequent is an older name for it.
Sources
- Affirming the Consequent, The Fallacy Files
- Fallacies, Bradley Dowden, Internet Encyclopedia of Philosophy
In the app
Practice spotting it on arguments that sound real.
Steelman has more examples of affirming the consequent, how to answer it in a real conversation, and practice rounds that mix it with its look-alikes, so you learn the difference, not just the name. Coming soon to iPhone and Android.
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