 The problems with the layer-cake account have been well-documented (Craver 2007: 172–6; Eronen 2015; Kim 2002; Potochnik &amp; McGill 2012; Rueger &amp; McGivern 2010). For one thing, if we look at contemporary science, the strict correspondence between levels and scientific fields breaks down immediately: Fields such as cognitive neuroscience span multiple levels, and the level of multicellular organisms is studied by a plethora of different scientific disciplines (Craver 2005; Bechtel 2008: 145). Likewise, the exhaustive stepwise condition on compositional relations between natural constituents, requiring that entities at one level are composed of only entities at the next lower level, is largely a caricature (e.g., Kim 2002): think of blood, nominally a tissue-level phenomenon, which is directly composed by molecular constituents such as vitamins and water without adjacent-level constituents, e.g., cells, playing any intermediary role (Brooks 2017). Finally, and most importantly, the layer-cake account aspires to a comprehensive notion of ‘levels’ that is wildly at odds with the way that ‘levels’ is actually used in science. Scientists often operate with a much more limited definition of the levels concept that either (a) is restricted to a specific and local context (see section 2.2) or (b) makes room for exceptions to what ‘levels’ expresses. Many biological sources in fact are completely aware that levels do not capture a completely uniform reality, and sometimes remark on important exceptions to the rules that are laid down by the levels concept. This is most prominently seen when referring to “organisms” in a piecemeal way so as to capture both multicellular and unicellular forms of life (Mader 2010: 2).