Abstract

The notion of a hypothesis being deployed essentially in the derivation of a novel prediction plays a key role in the deployment realist reply to Laudan’s and Lyon’s attacks to the No Miracle Argument. However Lyons criticized Psillos’ criterion of essentiality, urging deployment realists to abandon this requirement altogether and accept as true all the assumptions actually deployed in novel predictions. But since many false assumptions were actually deployed in novel predictions, he concludes that the “no miracle argument” and deployment realism fail. I reply that the essentiality condition is required by Occam’s razor, and a “no miracle argument” deprived of that condition is a straw man. While Lyons’ criticisms of Psillos’ formalization of essentiality may be well taken, I suggest that there is a simpler formulation which escapes those criticisms and is enough to deal with the supposed counterexamples to the No Miracle Argument and deployment realism: a hypothesis is essential when it does not have a proper part (in Yablo’s sense) from which the prediction could also be derived. I then show how this condition rescues deployment realism from Lyons’ purported historical counterexamples. Essentiality in this sense cannot be detected prospectively, but this is just what we should expect, and far from causing problems for deployment realists, this frees them from unreasonable obligations.