The 20th century saw some steps away from definitions toward alternative views of concepts, notably prototypes, exemplars, and theories (Machery 2009). Prototype concepts are abstract features shared by most, but not all members of a category (Rosch &amp; Mervis 1975; Rosch 1978; Hampton 1979, 2006; Smith 2002). The definitions of life in biology textbooks might be charitably understood as prototype concepts.  So, too, are the property cluster natural kinds popular in philosophy of biology (Boyd 1991, 1999, 2010; Diguez 2013; Slater 2015). Exemplars are concepts built around similarity to a particular individual case (Medin &amp; Schaffer 1978, Nosofsky 1986). Both prototypes and exemplar concepts rely on similarity to paradigmatic cases, with the former being an imagined ideal and the latter being a real instance (Komatsu 1992). For similarity-based concepts to work in scientific cases such as life, we need an account of which similarities matter, how much, and why. In contrast to similarity-based concepts, the requirements of theory concepts are somewhat more nebulous. Theory concepts are modeled on scientific theories and thus reflect their diversity (Carey 1985, Murphy &amp; Medin 1985, Gopnik &amp; Meltzoff 1997). At the core, theory concepts rely on explanations for why the members of a category share certain properties. Marginal cases of life, such as viruses, prions, or protocells, might be included in some theories of life but not others.