 Since Nagel’s influential model of reduction by derivation most discussions of unity of science have been cast in terms of reductions between concepts, the entities they describe, and between theories incorporating the descriptive concepts. Ontological unity is expressed by a preferred set of such ontological units. In terms of concepts featured in preferred descriptions, explanatory or not, reduction endorses taxonomical monism, a privileged set of kinds of things. These privileged kinds are often known as so-called natural kinds, although the notion admits of multiple interpretations, ranging from the more conventionalist to the more essentialistic. Regardless, the fundamental units are ambiguous with respect to their status as either entity or property. Reduction may determine the fundamental kinds or level through the analysis of entities. A distinctive ontological model is this: The hierarchy of levels of reduction is fixed by part-whole relations. The levels of aggregation of entities run all the way down to atomic particles and field parts, rendering microphysics the fundamental science.