The quality of our public discourse – think of the climate change debate for instance – is never very high. A day spent observing it reveals a litany of misrepresentation and error, argumentative fallacy, and a general lack of good will. In this paper I focus on a microcosmic aspect of these practices: the use of two types of argument – the argumentum ad hominem and appeal to authority – and a way in which they are related. Public debate is (...) so contaminated by the misuse of the ad hominem tactic that it is important to have an understanding of one of the ways in which its use may be legitimate. So readers who might have expected an analysis of the ad hominem fallacy will, on the contrary, be presented with an account of one of the places for its potential success. In ad hominem, arguers are viewed as legitimate constitutive targets for their own argument; in authority arguments, arguers provide a legitimate constitutive warrant for an argument they deploy. These types of argument are connected in so far as the identities of persons deploying them carry the cognitive weight of the argument. If authority arguments sometimes deserve to succeed, ad hominem attacks on them are potentially legitimate. An important motivation for this analysis is that in identifying the success conditions for one of the ad hominem types we attain some clarity in the evaluation of their use. (shrink)