 In this framework, O&amp;P suggest ordering the branches of sciences so that the major potential reductions standing between the current situation and unified science can be identified. For this purpose, they propose six “reductive levels” (1958: 9): Social groups; (Multicellular) living things; Cells; Molecules; Atoms; Elementary particles. The branch of science with the things of a given level as its universe of discourse is a potential microreducer of a branch with the things of the next higher level as its universe of discourse (O&amp;P 1958: 9).