 Historically prominent philosophers of science such as Popper and Reichenbach rejected inquiries into how scientific theories are discovered as not philosophical (see the entry on  scientific discovery).  Beginning with Hanson (1958), though, discovery has attracted the interest of many philosophers of science. Investigations into discovery in cell biology (and related fields such as biochemistry, molecular biology, and neuroscience) inspired Bechtel and Richardson (1993 [2010]) to argue that the goal in these fields was not to construct nomological explanation (Hempel 1965) but rather to identify the mechanism responsible for a phenomenon and determine how it worked. Examples from cell biology have also figured prominently in accounts of mechanistic explanation by Machamer, Darden, and Craver (2000), Craver and Darden (2013), Bechtel (2006), and others (see Glennan &amp; Illari, 2018, for a compendium of contemporary accounts of mechanisms and mechanistic explanation, and the entry  mechanisms in science).  The concept mechanism figures both in discussion of ontological issues and epistemic issues, but the two uses can be distinguished in the manner proposed by G. Allen (2005). In this section we are concerned with what he called “operative or explanatory mechanism” (2005: 261; the thesis that for explanatory purposes components of cells should be conceived to function as if they were machines);  Section 3  concerned “philosophical mechanism” (2005: 261; the ontological thesis that organisms are or are constituted by machines).