Update In reading over Professor Salmon s Introduction to Logic and Critical Reasoning I have found a passage which appears quite clearly to support my position that UD s example is an ad hominem as opposed to an argument from authority Professor Salmon analyzes a dispute between two scientists using it to give practical examples of informal fallacies Consider the following example Dr Alvarez responds by saying that he considers Dr Clemens inept at interpreting sedimentary rock strata and that his criticisms can be dismissed on grounds of general incompetence a charge Dr Clemens rejects Professor Salmon classifies this example as an abusive ad hominem p 125 and it appears to be of the same form as UD s example i e 1 X says Y 2 X is incompetent 3 Therefore Y I believe that this really ought to put the matter to rest unless someone can find a source supporting Mr Richfield s position in which case it might be helpful to include both views while noting the dispute