Scott Woodcock has levied a number of objections against Aristotelian naturalism which claims that ethical norms are grounded by reason and biology. His most recent “membership objection” is a synthesis of earlier objections and consists in a trilemma. If Aristotelian naturalists answer the first horn of the trilemma by stipulating that determinations of species-membership are grounded non-empirically, and the second horn of the trilemma by stipulating rationality is species-specific, then they are confronted by a moral outsider—someone who claims that they (...) have non-empirically determined their species membership and are thus guided by different norms of rationality than the rest of us. This permits the moral outsider to act heinously without moral sanction from Aristotelian naturalism. Critics have neglected Alasdair MacIntyre’s Aristotelian naturalism. And he has faced moral-outsider-type counterexamples before. I develop a new response to counterexamples MacIntyre’s account has faced and argue that MacIntyre’s Aristotelian naturalism is able to answer Woodcock’s membership objection. I do this by developing an empirical taxonomy-based approach that enables us to justify the moral outsider’s responsibilities while preserving the best of Aristotelian naturalism. (shrink)