Yet the contrast between laws and mechanisms has not always been entirely clear. Some, such as Bogen (2005), Machamer (2004), and Glennan (forthcoming) emphasize that causes and mechanisms are, at base, singular, not general or universal. Leuridan (2010), building on the work of Mitchell (2000), objects that mechanisms cannot replace laws of nature in our conceptual understanding of explanation and the metaphysics of science. Scientists rarely investigate token mechanisms, one might think, but are much more interested in types. And once one starts talking about types of mechanisms, one is back in the business of formulating general regularities about how mechanisms work. So it would appear that the concept of mechanism cannot supplant the work that generalization was supposed to do, but requires the idea of regularity, and so something akin to laws, if it is to do that explanatory work (see Andersen 2011, 2012, 2014a,b; Krickel 2014). For a reply to Leuridan, see Kaiser and Craver (2013).  