Explanatory Generalizations, Part II: Plumbing Explanatory Depth

Some explanations are deep and powerful: Newton's explanation of the tides, Maxwell's explanation of the propagation of light, Einstein's explan- ation of the advance of the perihelion of Mercury. Other explanations, while deserving of the name, are superficial and shallow: Bob lashed out at Tom because he was angry, the car accelerated because Mary depressed the gas pedal with her foot, the salt dissolved because it was placed in water. We take this intuition to be very natural and widely shared. Yet in the vast philosophical literature on explanation, there have been precious few attempts to give any systematic account of this notion of explanatory depth. In this paper, we will provide such an account fromwithin the framework of the manipulationist account of explanation presented in a companion paper (Woodward and Hitchcock 2003, hereafter referred to as EG1; see also Woodward 1997a, 2000). We believe that the absence of any adequate theory of explanatory depth is no accident. According to most theories of explanation, explanations appeal (at least tacitly) to generalizations of some sort. For example, in Hempel's Deductive-Nomological (D-N) theory of explanation (Hempel and Oppenheim 1948, Hempel 1965a), explanations must appeal to true, lawlike generalizations—i.e., to laws. A generalization is a proposition that is general in the following sense: it describes more than just the actual properties of the particular systemthat is the focus of explanation. This suggests a natural approach to the problemof explanatory depth: an explanation is deeper insofar as it makes use of a generalization that is more general. We will ultimately endorse a version of this strategy. We will