 The second problem concerns the nature of absolute space. Newton quite clearly distinguished his account from Descartes’ – in particular with regards to absolute space’s rigidity versus Descartes’ ‘hydrodynamical’ space, and the possibility of the vacuum in absolute space. Thus absolute space is definitely not material. On the other hand, presumably it is supposed to be part of the physical, not mental, realm. In De Gravitatione, Newton rejected both the traditional philosophical categories of substance and attribute as suitable characterizations. Absolute space is not a substance for it lacks causal powers and does not have a fully independent existence, and yet it is not an attribute since it would exist even in a vacuum, which by definition is a place where there are no bodies in which it might inhere. Newton proposes that space is what we might call a ‘pseudo-substance’, more like a substance than property, yet not quite a  substance.[7]  In fact, Newton accepted the principle that everything that exists, exists somewhere – i.e., in absolute space. Thus he viewed absolute space as a necessary consequence of the existence of anything, and of God’s existence in particular – hence space’s ontological dependence. Leibniz was presumably unaware of the unpublished De Gravitatione in which these particular ideas were developed, but as we shall see, his later works are characterized by a robust rejection of any notion of space as a real thing rather than an ideal, purely mental entity. This is a view that attracts even fewer contemporary adherents, but there is something deeply peculiar about a non-material but physical entity, a worry that has influenced many philosophical opponents of absolute  space.[8]