Should an argument be considered invalid if there is an unnecessary premise?

P1: A dog is a mammal. P2: All mammals have hair. P3: A dog is an omnivore. C: A dog has hair.

1 Answer 1


An extra premise does not make an argument invalid.

Valid = it is not possible for all of the premises to be true and the conclusion false.

Considering the argument's conclusion is true whenever P1 and P2 are true, it is impossible for P1, P2, and P3 to all be true and the conclusion false.

The problem with extraneous premises is that they affect the clarity of an argument. Specifically, they make it "verbose" rather than concise. But this is a rule of style rather than something that affects validity.

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