ABSTRACT To modern scholars, the naturalistic fallacy looks out of place in Greco-Roman antiquity owing to the robust associations between nature, especially human nature, and moral norms. Yet nature was understood by ancient authors not only as a norm but also as a form of necessity. The Greco-Roman philosophical schools grappled with how to reconcile the idea that human nature is given with the idea that it is a goal to be reached. This essay looks at the Stoic concept of (...) oikeiōsis as one strategy for effecting such a reconciliation. Drawing on natural history, these Stoic sources used examples of animal behavior to illustrate a process whereby nature “entrusts” all animals, including humans, with the care of their own survival. Nature is thus both what is given to the animal and what the animal achieves in a powerful but also problematic synthesis here called the “naturalistic fantasy.”. (shrink)