Philosophers of biology claim that function talk is consistent with naturalism. Yet recent work in biology places new pressure on this claim. An increasing number of biologists propose that the existence of functions depends on the organisation of systems. While systems are part of the domain studied by physics, they are capable of interacting with this domain through organising principles. This is to say that a full account of biological function requires teleology. Does naturalism preclude reference to teleological causes? Or (...) are organised systems precisely a naturalised form of teleology? In this paper I suggest that the biology of organised systems reveals several contradictions in the main philosophical conceptions of naturalism. To integrate organised systems with naturalism’s basic assumptions—that there is no theory-independent view for metaphysics, and that nature is intelligible—I propose an idealist solution. (shrink)