Comment by spankibalt

If you play the messenger deductively, then its invalid. So much for formal logic. Most arguments in practice are defeasible, and the ad hominem, for better or worse, often plays a significant and essential part in their resolution (addressing reputations, biases, agendas, interest conflicts, and so on). Virtually all interactions between people, even formal ones (e. g. in courts), are not some school exercise in formal logic.