Temporal parts and their individuation
1. Ignoring the temporal dimension, an object such as a railway tunnel or a human body is a three-dimensional whole composed of threedimensional parts. According to a rather natural point of view, if it is true that (for example) a tunnel has the property of running due north at the Somerset-Dorset boundary, this is because the tunnel has a spatial part that straddles the county boundary and is oriented north-south. The fourdimensionalist holds that a physical object exhibiting identity across time Descartes, for example is a four-dimensional whole composed of 'briefer' four-dimensional objects, its temporal parts. According to the four-dimensionalist, if Descartes has the property of, say, having fought a duel (at t), this is because Descartes has a temporal part (his t-part) that is fighting a duel. Peter van Inwagen (1990) has argued that four-dimensionalism cannot be sustained, or at best can be sustained only by a counterpart theorist. His argument against four-dimensionalism is simple (1990: 253):