Abstract

Substantialism with respect to space, time and space-time is the view that they are physical entities which exist in their own right, and are ontologically prior to other physical objects. Relationism, by contrast, is the view that insofar as space, time and space-time do exist they are some sort of construct out of ordinary physical objects and so are ontologically dependent on them. But why believe in relationism? ;A good argument for relationism was given by Liebniz. If substantival space exists, then states of the world which differ only over the position of the world in space are different states of the world. But such different states are indiscernible, in conflict with the identity of indiscernibles. So substantival space does not exist. ;A non-trivial version of the identity of indiscernibles is defended: no individuals are numerically distinct unless they differ over some basic qualitative property. The principle is also defended against counter-examples to it from the possibility of symmetric worlds by arguing that in such cases it is more plausible to suppose that the world has an alternative geometry and is not symmetric than that the principle is false. ;But although the identity of indiscernibles is a plausible premise in Leibniz's argument, his conclusion does not follow unless one assumes that the substantivalist is committed to haeccietism. Yet nothing in the substantivalist position commits him to haeccietism, and haeccietism is independently implausible on epistemological grounds. ;So Leibniz's argument against substantivalism fails. But there are also good reasons for believing in substantivalism. Physical space-time is required in causal explanations of accelerations. Only on the basis of physical space-time can we explain quantitative relations amoung objects in terms of qualitative relations among space-time points. And physical space-time is causally efficacious; electromagnetic and gravitational field strengths are properties of space-time points.