Newton's anti-cartesian considerations regarding space

In De Gravitatione, Isaac Newton makes it quite clear what he thinks space is not. In particular, he raises objections to René Descartes's construal of space as extended substance. However, central features of his positive view remain unclear. Consider, for example, the following key passage:
Perhaps now it may be expected that I should define extension as substance, or accident, or else nothing at all. But by no means, for it has its own manner of existing which is proper to it and which fits neither substances nor accidents. It is not a substance: on the one hand, because it is not absolute in itself, but is as it were an emana tive effect of God and an affection of every kind of being; on the other hand, because it is not among the proper affections that denote sub stance, namely actions, such as thought in the mind and motions in the body.