 In the classic paper “The Unity of Science as a Working Hypothesis” (1958; see also the entry  the unity of science),  Oppenheim and Putnam (hereafter O&amp;P) put forward a system of levels that became very influential, and is still implicitly present in most references to levels. This “layer-cake” model comprises three components. First, their conception of levels was comprehensive, meaning they wished to subsume all instances of where one could talk of ‘levels’ into their account. Second, O&amp;P posited that levels are related via compositional relations that are structured in a stepwise fashion. That is, all constituents of the objects of study of one branch of science, or, the branch’s “universe of discourse,” are exhaustively related as wholes to the parts located at the next adjacent lower level, and as themselves parts to the constituents occurring at the next adjacent higher level. This component is probably most responsible for the “layer-cake” moniker, which continues to appear in discussions of levels even today. Finally, O&amp;P presumed a strict correspondence between the constituents comprising a level and the predicates and theories linked with these constituents, meaning that levels of science neatly map onto levels of nature, so that for each level in nature there is corresponding science or theory and vice versa (Craver 2007: 174–175; Brigandt 2010: 304–305).