On the Persistence of Homogeneous Matter

Abstract

Some recent philosophical debate about persistence has focussed on an argument against perdurantism that discusses rotating perfectly homogeneous discs (the `rotating discs argument'; RDA). The argument has been mostly discussed by metaphysicians, though it appeals to ideas from classical mechanics, especially about rotation. In contrast, I assess the RDA from the perspective of the philosophy of physics. After introducing the argument and emphasizing the relevance of physics (Sections 1 to 3), I review some metaphysicians' replies to the argument (Section 4). Thereafter, I argue for three main conclusions. They all arise from the fact, emphasized in Section 2, that classical mechanics (non-relativistic as well as relativistic) is both more subtle, and more problematic, than philosophers generally realize. The main conclusion is that the RDA can be defeated (Section 6 onwards). Namely, by the perdurantist taking objects in classical mechanics (whether point-particles or continuous bodies) to have only temporally extended, i.e. non-instantaneous, temporal parts: which immediately blocks the RDA. Admittedly, this version of perdurantism defines persistence in a weaker sense of `definition' than {\em pointilliste} versions that aim to define persistence assuming only instantaneous temporal parts. But I argue that temporally extended temporal parts are supported by both classical and quantum mechanics.