2.2.26

The foundation of the system is that the universe - the 'all viewed as one' - has two real constituents: body and void space. The existence of body is proved to us by sensation; if we cannot know by sensation that there are bodies, then we cannot know anything at all. But bodies have location, and they move. That is to say, the universe is not as it might have been - solid, packed tight, immobile. In brief: 'All nature . . . is built of those two things: for there are bodies, and there is the void in which they are placed and where they move' (Lucretius, De rerum natura, Book I). 

– John Gaskin, The Travelers Guide to Classical Philosophy