Steelman
Exclusive premises
Also called two negative premises.
Two negative premises can never support a conclusion.
What it is
A categorical syllogism in which both premises are negative (“No…” or “Some… are not…”). No valid syllogism has two negative premises.
The pattern
No M are P. No S are M. Therefore, no S are P. (invalid; any syllogism with two negative premises is)
An example
“No fish are mammals. No whales are fish. Therefore, no whales are mammals.”
Both premises are true and the conclusion is false: whales are mammals.
Why it fails
Each negative premise only says what is kept apart from the middle term. Knowing that S and P are both excluded from M tells you nothing about how S and P relate to each other.
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
- This rule is specific to categorical syllogisms. Other valid arguments can contain two negative statements, e.g. “If it’s not raining, we won’t cancel. It’s not raining. So we won’t cancel.”
Try one
A question from the app. The wrong answers are its usual look-alikes.
Team group chat. Marcus: “Good news for the opener. Coach sat every starter in Saturday’s preseason game, so none of them played. And the trainer says nobody who played Saturday is on the injury report. So none of our starters are on the injury report. Full strength, baby. Bring on Sunday.”
What’s the flaw?
Both facts say who isn’t somewhere, and both are about Saturday’s game. They can’t tell you anything about a starter who got hurt some other way, like in practice.
A fair reply
“Saturday’s game isn’t the only way to get hurt. Did anyone tweak something in practice?”
Often confused with
- Affirmative conclusion from a negative premise
That fallacy draws a positive conclusion from a negative premise. Exclusive premises is about having two negatives at all.
Where the name comes from
Premises are the statements an argument starts from. Exclusive comes from the Latin for “shut out,” and here both premises only shut one group out of another. Two shut-outs never say how the remaining groups relate to each other. The other name, two negative premises, says the same thing: both say “no” or “are not.”
Sources
- Exclusive Premisses, The Fallacy Files
- Syllogistic Fallacy (terminology and distribution), The Fallacy Files
In the app
Practice spotting it on arguments that sound real.
Steelman has more examples of exclusive premises, 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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