The central claim of Aristotelian naturalism is that moral goodness is a kind of species-specific natural goodness. Aristotelian naturalism has recently enjoyed a resurgence in the work of philosophers such as Philippa Foot, Rosalind Hursthouse, and Michael Thompson. However, any view that takes moral goodness to be a type of natural goodness faces a challenge: Granting that moral goodness is natural goodness for human beings, why should we care about being good human beings? Given that we are rational creatures who (...) can ‘step back’ from our nature, why should we see human nature as authoritative for us? This is the authority-of-nature challenge. In this essay, I state this challenge clearly, identify its deep motivation, and distinguish it from other criticisms of Aristotelian naturalism. I also articulate what I consider the best response, which I term the practical reason response. This response, however, exposes Aristotelian naturalism to a new criticism – that it has abandoned the naturalist claim that moral goodness is species-specific natural goodness. Thus, I argue, Aristotelian naturalists appear to face a dilemma: Either they cannot answer the authority-of-nature challenge, or in meeting the challenge they must abandon naturalism. Aristotelian naturalists might overcome this dilemma, but doing so is harder than some Aristotelians have supposed. In the final sections of the paper, I examine the difficulties in overcoming the dilemma, and I suggest ways that Aristotelians might answer the authority-of-nature challenge while preserving naturalism. (shrink)