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Full Idea
It is a good argument ad hominem, if it can be shewn that a first principle which a man rejects, stands upon the same footing with others which he admits, …for he must then be guilty of an inconsistency.
Gist of Idea
An ad hominem argument is good, if it is shown that the man's principles are inconsistent
Source
Thomas Reid (Essays on Intellectual Powers 6: Judgement [1785], 4)
Book Ref
Reid,Thomas: 'Inquiry and Essays', ed/tr. Beanblossom /K.Lehrer [Hackett 1983], p.261
A Reaction
Good point. You can't divorce 'pure' reason from the reasoners, because the inconsistency of two propositions only matters when they are both asserted together. …But attacking the ideas isn't quite the same as attacking the person.