Argumentum ad hominem

Your claim that everyone should become a vegetarian makes no sense. You work at a seafood restaurant!

In this argument, the unacceptability of the claim that “Everyone should become a vegetarian” is supported by pointing out an inconsistency between the source’s belief in this claim and their behavior. Interpreting the second sentence as a premise supporting the lever rather than the premise supporting the conclusion directly, the argument form can be determined as delta (q is U because q is Z): “Everyone should become a vegetarian (q) is unacceptable (U) because everyone should become a vegetarian (q) is said by you (Z)”.

The premise takes a second-person perspective, so the argument substance is ‘II’: “Everyone should become a vegetarian is unacceptable because everyone should become a vegetarian is said by you (II)”.

The keyword CHARACTERALLY AUTHORITATIVE describes the relationship between predicates Z and U. The argument lever can thus be formulated as “Being said by you (Z) is CHARACTERALLY AUTHORITATIVE for being unacceptable (U)”.

Source

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Notes

The second sentence of the example expresses the fact that the source of the claim works in a seafood restaurant and can be reconstructed as a premise that supports the lever of the original argument: “Being said by you is CHARACTERALLY AUTHORITATIVE for being unacceptable because you work at a seafood restaurant”.

The tradition distinguishes three variants of the argumentum ad hominem: (1) the direct or abusive ad hominem, which attributes a negative characteristic to the source, (2) the indirect or circumstantial ad hominem, which doubting the motives fort he source to endorse the claim, and (3) the tu quoque, which points out an inconsistency between the source’s behaviour and their believing the claim.

While the tradition reserves the label ad hominem for attacks of the other party in the discussion, and thus with the second-person perspective, there is no reason why it could not also be used for arguments in which the unacceptability of a claim is supported by attacking a third party. In this case, the argumentum ad hominem has substance ‘III’ and can be interpreted as a negative argument from authority. Instead of the acceptability of a claim, the arguer is supporting the unacceptability of a claim by referring to its source.