Steelman
Affirmative conclusion from a negative premise
Also called illicit negative.
A premise says what things are not, yet the conclusion says what they are.
What it is
A categorical syllogism with at least one negative premise but an affirmative conclusion. Every valid syllogism with a negative premise has a negative conclusion.
The pattern
No M are P. All S are M. Therefore, all S are P. (invalid)
An example
“No dogs are cats. All puppies are dogs. Therefore, all puppies are cats.”
The valid conclusion from these premises is negative: “No puppies are cats”.
Why it fails
A negative premise separates two groups. From a separation you can only validly conclude another separation, never an inclusion.
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
- The conclusion is negative, e.g. “No M are P. All S are M. Therefore, no S are P.” That form (Celarent) is valid.
Try one
A question from the app. The wrong answers are its usual look-alikes.
“No bird is a mammal, and every penguin is a bird. It follows that every penguin is a mammal.”
What’s the flaw?
A “no” statement separates two groups, so all it can support is another separation. What follows here is the opposite: no penguin is a mammal.
A fair reply
“Those two lines prove the reverse: no penguin is a mammal.”
Often confused with
- Exclusive premises
Exclusive premises has two negative premises. This fallacy needs only one, paired with a positive conclusion.
Where the name comes from
An affirmative statement says what something is: “All starters are fast.” A negative one says what something isn’t: “No starters are slow.” This fallacy draws an “is” conclusion from premises where at least one says “isn’t.” Its other name, illicit negative, points at the culprit: a “no” premise put to a use the rules don’t allow.
Sources
- Affirmative Conclusion from a Negative Premiss, 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 affirmative conclusion from a negative premise, 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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