Affirmative conclusion from a negative premise is a logical fallacy that is committed when a categorical syllogism has a positive conclusion, but one or two negative premises.

For example:

No fish are dogs, and no dogs can fly, therefore all fish can fly.

This could be illustrated mathematically as

If A ? B and B ? C then A ? C.

It is a fallacy because any valid forms of categorical syllogism that assert a negative premise must have a negative conclusion.

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