 At the beginning of the 20th century, several notions of “levels” began to circulate in the philosophical and scientific literature. Prominent among these was the idea of “levels of existence” (alternatively: “being”; “complexity”) developed by philosophers Samuel Alexander (1920: 3, 45) and Charles Dunbar Broad (1925), which figured prominently in the emergentist and emergent evolution literature of the 1920’s (Sellars 1926; McLaughlin 1992; Stephan 1999). This introduced a number of themes that have become germane to general levels-language. These themes include level-specific laws (Broad 1925: 77; cf. Woodger 1929) and the idea that ‘levels’ exhibit a tendency in nature towards increasing complexity (Sellars 1917: 224; cf. Needham 1937), Although most of these themes were severely underdeveloped and divorced from the cutting-edge work in the life sciences happening at the time (Needham 1937: 242 ff. 4), this tradition has been an influential historical source for explicating levels language in philosophy of mind (see the entries on  emergent properties  and  supervenience;  McLaughlin 1992; Beckermann et al. 1992; Kim 1999, 2002).