Steelman
Illicit conversion
Also called false conversion.
“All A are B” does not mean “all B are A”.
What it is
Swapping the subject and predicate of an “All S are P” or “Some S are not P” statement and treating the result as equivalent. Conversion is only valid for “No S are P” and “Some S are P”.
The pattern
All S are P. Therefore, all P are S. (invalid) Some S are not P. Therefore, some P are not S. (invalid) Valid: No S are P ⇔ No P are S. Some S are P ⇔ Some P are S.
An example
“All surgeons are doctors, so all doctors are surgeons.”
Surgeons are a subset of doctors. Most doctors aren’t surgeons.
Why it fails
“All S are P” puts all of S inside P, but P can be much bigger. Only the symmetric statements (“no overlap”, “some overlap”) read the same in both directions.
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
- Converting “No S are P” or “Some S are P”, which are symmetric and valid to reverse.
Try one
A question from the app. The wrong answers are its usual look-alikes.
Kai, in the group chat: “Anything with pro leagues, paid players, coaches and a world championship is a sport. Think about what every real sport has: those four things. Football, tennis, cricket, all of them. League has every one of those.” Rhea: “League’s a video game. Calling it a sport is a stretch.”
What’s the flaw?
Kai starts from “every sport has these things” and flips it into “everything with these things is a sport”. The first puts sports inside the group; it doesn’t put the whole group inside sports. Poker has all four too.
A fair reply
“Poker has pro tours, paid players, coaches and a world championship. Is poker a sport?”
Often confused with
- Confusing sufficient and necessary
The same reversal for if-then statements: “if P then Q” doesn’t give “if Q then P”.
Where the name comes from
Conversion means swapping the two groups in a statement, so “All A are B” turns into “All B are A.” The word comes from the Latin for “turning around.” Illicit means not allowed: the swap only holds for “No A are B” and “Some A are B.” False conversion is the same idea: a swap that doesn’t hold.
Sources
- Illicit Conversion, 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 illicit conversion, 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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