 Consider the case of Interstate 5. I-5 runs through Washington, Oregon, and California, but the road is not wholly present in any of those states. Rather, I-5 exists in different states by having different road segments in each—there is the Washington segment of I-5, the Oregon segment, etc. According to the temporal parts theory (or four-dimensionalism), persistence through time is exactly like that. (Quine 1953, Lewis 1976, Sider 2001) Just as roads exist at different places by having distinct spatial parts at those places, material objects exist at different times by having distinct temporal parts at those times. In the case of David Lewis, for example, there is the 1970s segment, the 1980s segment, and so on. More formally, we can say that x is a temporal part of y at (or during) t if and only if (i) x is a part of y at (or during) t, (ii) x overlaps everything that is a part of y at (or during) t, and (iii) x exists only at (or during) t (Sider 2001, p. 60).