Steelman
Existential fallacy
Also called fallacy of existential assumption.
Concluding that something exists from premises that only say what would be true of it if it did.
What it is
Drawing a conclusion that implies some group has members (“Some S are P”) from premises that don’t say the group has any members, typically going from universal premises (“All…”, “No…”) to a particular conclusion.
The pattern
All S are M. All M are P. Therefore, some S are P. (invalid if there might be no S)
An example
“All perpetual-motion machines break the laws of physics. Everything that breaks the laws of physics is impossible to build. So some perpetual-motion machines are impossible to build.”
“Some perpetual-motion machines…” implies that such machines exist, but the premises never said so, and they don’t.
Why it fails
In modern logic, “All S are P” means “anything that is an S is a P”. It’s true even if there are no Ss at all (“all trespassers will be prosecuted” can be true when nobody trespasses). “Some S are P” claims at least one S exists. You can’t get existence out of premises that never asserted it.
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
- Everyone knows the group has members, so an unstated premise “there are some S” can fairly be assumed.
- You’re using traditional Aristotelian logic, which assumed every term refers to something. Under that assumption these forms were counted as valid. Modern (Boolean) logic dropped the assumption.
Try one
A question from the app. The wrong answers are its usual look-alikes.
“At least some dragons are dangerous, and here’s why. Every dragon breathes fire, and anything that breathes fire is dangerous, so every dragon is dangerous.”
What’s the flaw?
“Every” statements don’t claim that any dragons exist, but “some dragons are dangerous” does. What validly follows is “every dragon is dangerous”, which is true even if there are no dragons.
A fair reply
“Those lines never said dragons exist; only ‘every dragon is dangerous’ follows.”
Often confused with
- Illicit minor
Illicit minor over-generalizes the subject. The existential fallacy under-supports existence.
Where the name comes from
Existential means having to do with whether something exists. The slip is concluding that some members of a group exist when the premises never said the group has any. Fallacy of existential assumption names the hidden step: assuming, without saying so, that the group isn’t empty.
Aristotle’s logic assumed every term names something that exists. Later-19th-century logicians following George Boole dropped that assumption, which is why some classic syllogisms now count as instances of this fallacy.
Sources
- The Existential Fallacy, The Fallacy Files
In the app
Practice spotting it on arguments that sound real.
Steelman has more examples of existential fallacy, 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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