There is an obvious paradox in the theories of the Atomists. If knowledge comes to us by means of sensation, how are we to explain the atomist's knowledge of the basic propositions of his special theory: that the world consists of void, which is called 'the intangible', and atoms, which are said to be imperceptibly small? Epicurus' method was to set up a pair of contradictories - either there is a void or there is not, either matter is infinitely divisible or it is not - and then to present an argument for rejecting one of them.
Furley, David J., "Knowledge of Atoms and Void in Epicureanism" (1967). The Society for Ancient Greek Philosophy Newsletter. 81.