 It is Naturalistic.  The natural world of space, time, and matter, with all its constituents, is a Reality in its own right.  Contrary to all Idealist metaphysics, this world, except for the finite minds that are to found within it, is independent of any knowing mind.  Moreover, it is the only world. There are no divinities or supernatural powers beyond the realm of Nature (“Naturalism and the Nature of Things” 1966, pp 212–238). Even mathematical realities belong in the natural world: numbers—at least natural ones—as abstractions from clusters, and geometrical objects as abstractions from space. The philosophy of mathematics was an aspect of his position that was never fully worked through.