There are four positions one might take in respect of the ontological status of the physical world: physicalism, which says that the physical world is ontologically fundamental, and nothing else is; substance dualism, which says that the physical world is ontologically fundamental, but so is the human mental realm, and that these are in some strong metaphysical sense separate; idealism, which says that the physical world is constitutively sustained, at least in part, by facts about the human

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... tal realm; and a rough collection of views I term 'compatibilism', which holds that both the physical and the mental are fundamental, but that they are not separate as in substance dualism. Of these positions, I argue mainly against the first and last. I begin by demonstrating that all forms of compatibilism are committed to a radically revisionary definition of 'mental' and 'physical', since in ordinary usage, and for good reason, the terms are taken as mutually exclusive. I formulate a definition of 'mental' according to which it means 'subjective, non-spatial, and non-quantifiable', and demonstrate that these properties are necessarily coextensive. Against physicalism, I consider a range of arguments which purport to show that physical space, as a necessary feature of the physical world, cannot be ontologically fundamental, concluding that physical space, or at least the physical space that we are interested in, must be the one which we inhabit, and that our relationship of inhabitancy of this physical space plays a constitutive role in it. Since this assumes that physical space must be in some way constituted rather than fundamental, I finish by refuting a set of strategies which attempt to show that physical space itself must be constituted.