Foster on matter and space
While the conceptions of the large-scale composition of the physical universe offered by substantivalists and relationists are undeniably very different, there is also a sense in which they do not differ at all: both camps accept that we inhabit a spatial world. The disagreement is limited to the question of how space (or spacetime) should be characterized. Despite their differences, substantivalists and relationists both count as spatial realists. In this chapter we will be examining a purely metaphysical argument for spatial anti-realism; the claim that our universe is not in fact spatial, despite appearances that suggest the contrary. The claim that space is unreal may well seem as absurd as McTaggart's assertion that time is unreal, but just as McTaggart's argument (or at least a close relative of it) proved to be both non-trivial and independent of idle sceptical considerations, so too will the argument we will be considering here.
In The Case for Idealism (1982) Foster develops an argument intended to establish that the physical world is not (and could not) be a part of what he calls “ultimate reality”, and the key step in reaching this conclusion is an argument for spatial anti-realism. Unfortunately, despite its interest, Foster's argument is complex, multi-faceted and often difficult; to do it full justice would require a lengthy exposition, and this is not the place for it.