Steelman
Quantifier shift
Also called illicit quantifier shift, every–some swap.
“Everyone has a mother” doesn’t mean “someone is everyone’s mother”.
What it is
Inferring “there is one Y that every X is related to” from “every X is related to some Y”, that is, swapping the order of “every” and “some”.
The pattern
Every X has some Y. Therefore there is some Y that every X has. (invalid) Valid the other way round: There is some Y that every X has. Therefore every X has some Y.
An example
“Every student in the class has a favorite song. So there’s one song that’s everyone’s favorite.”
Each student can have a different favorite.
Why it fails
“Every X has some Y” allows each X a different Y. The conclusion requires a single Y shared by all of them, which the premise never said.
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
- There’s independent reason to believe a single Y covers every X.
Try one
A question from the app. The wrong answers are its usual look-alikes.
Late-night dorm debate. Sam: “Each thing that happens has a cause, right? The rain, the tides, you and me.” Ana: “Sure, I’ll give you that.” Sam: “Then there’s one cause behind all of it, the same one for the rain, the tides and us. That’s what I mean by God.”
What’s the flaw?
From “each thing has a cause” Sam jumps to “one cause is behind all of them.” The first allows the rain and the tides to have different causes; it doesn’t give you a single cause they all share.
A fair reply
“Each thing having a cause doesn’t mean they all share the same one, so the rain’s cause and the tides’ cause could be different.”
Often confused with
- Fallacy of composition
Composition moves a property from parts to the whole. A quantifier shift reorders “every” and “some”.
- Amphiboly
Quantifier-scope ambiguity is a kind of amphiboly. The shift fallacy exploits it to draw a stronger conclusion.
Where the name comes from
Quantifiers are words that say how many, like “every” and “some”. The shift moves them around, so “every X has some Y” becomes “some Y belongs to every X”. “Illicit” means not allowed, since that swap isn’t valid. “Every–some swap” names the move in plain words.
Sources
- Quantifier-Shift Fallacy, 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 quantifier shift, 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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