What it is
From if A then B, and B is true, wrongly concluding A. Other causes of B are ignored.
In the wild
If it rains the road is wet; the road is wet, so it rained.
How to answer it
Name one other thing that would produce the same result.
Broken structure
Affirming the consequent
Also called Converse error
In depth
Origin and naming
Named for its shape: the consequent — the 'then' half of a conditional — is affirmed, and from that the 'if' half is concluded. It is the invalid twin of modus ponens, one of the two ways a hypothetical syllogism can misfire.
More places it shows up
If the servers are overloaded the site is slow; the site is slow this morning, so the servers must be overloaded — and nobody checks the office internet.
Candidates who bluff get caught in interviews; this candidate was caught, so he must have been bluffing.
If the new feature worked, users would upgrade; users upgraded, so the feature works.
When the price drops, demand rises; demand rose last quarter, so the price drop did it.
How to spot it
- Notice when the shape is 'if cause then effect; effect happened; so cause happened' — ask what else produces that effect.
- Notice when the argument never asks whether the stated condition was the only path to the result.
- Notice when swapping the explanation for another plausible one leaves the premises just as true.