  For simplicity, we stay with the principle as formulated at the outset. Why think that it’s true? Perhaps it is a conceptual truth: for an effect to be physical is, at least in part, for it to have a physical cause. This defense turns on the proper analysis of the concept physical, itself the subject of a contentious literature (see physicalism). Here we simply note that the principle does not seem analytic; it appears to be a substantive, empirical claim about the causal structure of the universe. (For more on the conceptual defense, see Crane 1991; Papineau 1991, 1993, §1.9; Lowe 1996, p. 56.)