 As for bodies (note the plural: Epicurus does not typically speak of matter per se) and void as the basic physical principles, the senses, Epicurus affirms, testify to the existence of bodies, and by calculation on the basis of the senses we infer the nature of what is invisible, for example the atoms (LH 39). Here the reasoning is based on analogy: what is evident to our senses must be true on the microscopic level as well, at least in some respects. Void must exist, in turn, if bodies are to be able to move, as they are seen to do. Thus motion is the counterwitness to the non-existence of void — an indirect argument is required since one cannot perceive empty space. What is more, since bodies, being “full,” offer resistance and void, being empty, offers no resistance, they complement each other and exhaust the possibilities; hence it is impossible to conceive of anything besides these two principles, apart from things that are accidents of them — accidents that arise from unions of elementary bodies in the void. (Inconceivability is another tool in Epicurus’ method of demonstration.) These elementary bodies, then, are the atoms, which are indivisible and inalterable, if things are not to dissolve into nothingness. The Letter to Herodotus is an epitome of Epicurean doctrine, and the arguments are crisp and abbreviated, but the reasoning is clear, and is confirmed by the more detailed treatment in Lucretius, which almost certainly follows Epicurus’ On Nature (see Leone 2012). Epicurus appeals to some elementary intuitions concerning bodies and their movement through space in order to establish the structure of imperceptibly small things; he concludes that these must be inalterable if nature is not to dissolve into nothing (creation back out of nothing having already been eliminated by the argument cited above from regularity in generation); and the basic features of the atomic system are then in place. A similar appeal to the senses establishes the infinity of the universe, since what is finite must have an edge, and an edge is conceived in reference to something beyond it. But the universe — in Greek, the “all” — contains everything, and so there is nothing outside it by which to conceive an edge. Hence, it is infinite. And if the all is infinite, so is the void and the number of atoms as well, for otherwise atoms would be too widely dispersed ever to meet (LH 41–42).