 Subsequent discussion based on ideas in metaphysics and  philosophy of mind  has clarified how one conception of the many-one relation between molecular and higher level kinds can bear on epistemic reduction. In his general treatment of reduction in psychology and social science, Jerry Fodor (1974, 1975, 1997) distinguishes between token-token reduction and type-type reduction (or between token physicalism and type physicalism). Token-token reduction is the idea that each token higher level property or process is metaphysically identical to a token physico-chemical property or process (the weak sense of ontological reduction that is uncontroversial: see  Section 1).  The dispute centers on how much more than token-token reduction is possible. A substantially stronger idea is that each type of higher level phenomenon corresponds to one type of lower level phenomenon. Type-type reduction yields a bold version of epistemic reductionism where higher level concepts and knowledge about higher level phenomena could be immediately reduced to molecular notions. Yet Fodor argues that type-type reduction (regarding mental and social kinds in particular) is empirically false. Consider a simple law or causal principle of a higher-level science relating two kinds, S and T, such that ∀x(Sx → Tx), which can be read as “all Ss are Ts” or “all events of type S bring about events of type T”. (Some laws or principles might be of a more complex logical form.) As a matter of fact, a higher level kind is often realized by several different kinds on the physical level; different instances of the higher level kind are of a different type on the molecular level. If kind S is realized by the lower level kinds M1, M2, …, Mi, the bridge principle (reduction function) is of a disjunctive form: