 Kemeny-Oppenheim reduction is understood as an indirect relation between a reducing theory (or branch of science) T1 and reduced theory (or branch) T2 relative to a set of observational data such that (a) both theories contain different vocabulary, (b) T1 explains least as much observational data as T2, but (c) T1 also “explains more” than T2 (1958: 5). The notion of a potential microreducer supplements this by capturing the part-whole coordination between the natural entities constituting the universes of discourse of different scientific theories or branches via exhaustive material compositional relations (1958: 6). This essentially provides an empirical mapping between the two scientific branches. Microreduction then combines these two, and thus amounts to a Kemeny-Oppenheim reduction between two scientific theories plus the fact that T1 is at a lower compositional level than T2.