“Top-Down” or “Bottom-Up”

Wesley Salmon has suggested that the two leading views of scientific explanation, the “bottom-up” view and the “top-down” view, describe distinct types of explanation. In this paper, I focus on theoretical explanations in physics, i.e., explanations of physical laws. Using explanations of E=mc2, I argue that the distinction between bottom-up explanations (BUEs) and top-down explanations (BUEs) is best understood as a manifestation of a deeper distinction, found originally in Newton’s work, between two levels of theory. I use Einstein’s distinction between ‘principle’ and ‘constructive’ theories to argue that only lower level theories, i.e., ‘constructive’ theories, can yield BUEs. These explanations, furthermore, depend on higher level laws that receive only TDEs from a ‘principle’ theory. Thus, I conclude that Salmon’s challenge to characterize the relationship between the two types of explanation can be met only by recognizing the close relationship between types of theoretical explanation and the structure of physical theory.