The conflation of the natural with the normative has counted as a philosophical fallacy for centuries. The oppositions of nomos and physis, is and ought, nature and culture all aim to drive a wedge between the inexorable facts of nature and the human values of ethics and art. This division is a principal feature of our philosophical and moral landscape. Yet the very frequency with which these oppositions must be invoked and insisted upon suggests that inferring from “what is the case” to “what ought to be the case” is still a deeply rooted habit of thought and feeling. At least within the Western tradition, the “naturalistic fallacy” has proved robustly resistant to philosophical harangues, and to a metaphysics of airtight categories that would prevent any mingling of the natural and the normative. Nature as ultimate source of moral authority surfaces in the most diverse contexts, some ancient, some modern, and others almost futuristic: the reproach “unnatural mother” is as old as the legend of Medea, but the notion of “natural human rights” is the invention of the Enlightenment, and the “unnaturalness” of human clones is a moral spectre conjured up in tomorrow’s newspaper. Appeals to the moral authority of nature are not restricted to popular culture: debates about nature as a standard for the good and the beautiful are also waged in science (e.g., in evolutionary theory) and law (e.g,. in environmental regulations). If the naturalistic fallacy is indeed a fallacy, it is a remarkably widespread and persistent one, and its resilience still requires historical explanation. The project "The Moral Authority of Nature" sought to do just this.