 This difficulty of justifying the inference from rats to humans or, more broadly, of “transferring causal generalizations from one context to another when homogeneity cannot be presumed” (Steel 2008: 3) is known as the problem of extrapolation. Although this problem is not unique to surrogate models, it often arises when biomedical researchers use them to replicate human disease at the molecular level. Consequently, philosophers who write about the problem of extrapolation in the context of molecular biology often focus on such models (see, for example, Ankeny 2001; Baetu 2016; Bechtel and Abrahamsen 2005; Bolker 1995; Burian 1993b; Darden 2007; LaFollette and Shanks 1996; Love 2009; Piotrowska 2013; Schaffner 1986; Steel 2008; Weber 2005; Wimsatt 1998).