Democritus and Leucippus, the fifth century atomists, argued that reality consists of indivisible microscopic particles called atoms. Leucippus was the first to make atoms first principles (Curd, 111). Their theory attempts to reconcile the rules of reality established by Parmenides and Xenophanes while explaining such apparent features as change and plurality.
In order to do this, Democritus and Leucippus treat atoms as what-is and void as what-is-not, which is no less than what is (Curd, 113). Void is the emptiness between atoms, and it functions as actualized non-being. Although the void is the absence of what is, it is nonetheless real. Hence they say that it is nothing less than what is. This does not seem a sufficient explanation of the void, however, because it attributes being to non-being.
Even so, they do explain "coming-to-be and perishing by means of separation and combination, alteration by means of arrangement and position" (Curd, 111). Atoms are all the same substance, but they differ in shape, arrangement, and position. This allows them constitute the plurality of things in the visible world while also being a unity. Nothing new truly comes into being or perishes, but rather matter changes form. This is not unlike the law of the conservation of energy in modern science, but the details are different. The atomists think that change comes from different arrangements of atoms as the atoms move through the void. Regardless, it is impressive that they posited a law of material conservation so early in the history of scientific inquiry.
Democritus and Leucippus provide a materialist explanation of reality in accord with Parmenides arguments. They posit the void in the place of what-is-not, which seems to present a contradiction by giving being to non-being. However, they do establish a law of material conservation that has some similarities with the law of the conservation of energy. Moreover, if they could articulate their concept of the void more fully, it is possible that their theory could explain change and plurality from the atomic first principle. Thus they increase our understanding of reality, though their theory needs refinement.