Parmenides argued that, because Nothing cannot be, multiplicity, movement, and change are all impossible. We observe multiplicity, movement, and change every day, though. By reductio ad absurdum, it appears that Nothing must be. And yet Nothing, by definition, is not.
How, then, can we affirm the possibility of multiplicity, movement, and change without simultaneously affirming a contradiction, i.e., that Nothing is?
The answer is quite simple. We have to affirm that empty Space is not Nothing. It is not Nothing, but empty Space that allows objects to be spatially separate from one another, and that also allows them to change position. Nothing is not, but empty Space is.
What, then, is Space?
Space is not a Thing, nor is it “stuff” (i.e. unformed matter, which is ultimately reducible to Things of some kind). If it were a Thing or “stuff,” then it could not be occupied by Things. When an object occupies Space, the Space is still there; it is not displaced. If Space were made of “stuff,” then this could not be so. Two things cannot occupy the same space, so if Space were “stuff,” then it could not be occupied.
We have difficulty conceiving of what is except in terms of Things and “stuff.” But Things and “stuff” do not exhaust the possibilities of Being. For instance, God is, yet he does not seem to be a Thing. Relations are, and they are not Things. Consciousness is, and it does not seem to be a Thing, either. Likewise, Space is, but it is not a Thing.
Of course, this raises the question: What is Space if not a Thing?
This is only the beginning of an answer, but it appears to me that the essence of Space is that it has the potential to be occupied by Things. In other words, Space is that within which Things can be. Empty Space still is, and it has the character of Space in that it can be occupied, even though it happens not to be occupied at a given time. Nothing, on the other hand, having no being, cannot be occupied. Where there is Nothing there is no potential for Things to be.
Space, then, is a necessary condition for the being of Things, as well as for the being of that which is derived from Things. But it is not a sufficient condition for Things to be. There can be Space without Things, but there cannot be Things without Space.