One thing of course was a fundamental necessity to the atomic world-view. There must be empty space for the atoms to move about in. The hallmark of Democritus’s thought, as Aristotle noted approvingly, was a determination to account for apparent fact and not be led astray by abstract argument. Hence he said that Parmenides’s denial of the existence of void could not be upheld. It was contrary to common sense. Aware however that he was flying in the face of that great authority, he made his denial with a kind of schoolboy daring, for according to Aristotle he put it in the form: ‘What is not does exist, no less than what is.’ If material atoms were the only real substance, then empty space was not real in the same sense. Dimly aware that there must be some way out, the atomists did not yet command a language capable of such a phrase as ‘not in the same sense’, and paradox was their only resource.