Inertial motion, explanation, and the foundations of classical spacetime theories

I begin by reviewing some recent work on the status of the geodesic principle in general relativity and the geometrized formulation of Newtonian gravitation. I then turn to the question of whether either of these theories might be said to "explain" inertial motion. I argue that there is a sense in which both theories may be understood to explain inertial motion, but that the sense of "explain" is rather different from what one might have expected. This sense of explanation is connected with a view of theories---I call it the "puzzleball view"---on which the foundations of a physical theory are best understood as a network of mutually interdependent principles and assumptions.