 (i) Ontological reduction is the idea that each particular biological system (e.g., an organism) is constituted by nothing but molecules and their interactions. In metaphysics this idea is often called  physicalism  (or materialism) and it assumes in a biological context that (a) biological properties  supervene  on physical properties (i.e., no difference in a biological property without a difference in some underlying physical property), and (b) each particular biological process (or token) is metaphysically identical to some particular physico-chemical process. This latter tenet is sometimes called token-token reduction, in contrast to the stronger tenet that each type of biological process is identical to a type of physico-chemical process. Ontological reduction in this weaker sense is a default stance nowadays among philosophers and biologists though the philosophical details remain controversial (e.g., are there genuinely  emergent properties?)  Different conceptions of physicalism may yield different implications for ontological reduction in biology (Dowell 2006). The denial of physicalism by vitalism, the view that biological systems are governed by forces that are not physico-chemical, is largely of historical interest. (Vitalism also admits of various conceptions, especially with respect to how non-physico-chemical forces are understood; see  Section 2.)  Some authors have argued vigorously for the significance of metaphysical concepts in discussions of reductionism in biology (Rosenberg 1978, 1985, 1994, 2006).