In this paper I provide support for the view that at least some forms of adversariality in argumentation are legitimate. The support comes from Aristotle's theory of illegitimate adversarial argumentation in dialectical contexts: his theory of eristic in his work On Sophistical Refutations. Here Aristotle develops non-epistemic standards for evaluating the legitimacy of dialectical procedures, standards which I propose can be understood in terms of the pragmatic notion of context as common ground. Put briefly, Aristotle makes the answerer's meaning in giving assent in dialectical contexts the basis for further moves in the game of dialectic. Moves which subvert the answerer's meaning or do not solicit the answerer's consent are marked as eristic, i.e. adversarial in a problematic sense. I conclude with remarks on what Aristotle's theory may teach us about how semantic features relate to the normative evaluation of argumentation.