 Epicurus operated with a highly limited number of elementary principles in nature — he did not know the concept of force, for example, or the associated ideas of attraction and repulsion among atoms, not to mention more arcane properties — and for all his efforts to account for all the physical features of the world on the basis of this theory, paradoxes remained. An excellent one is posed by Sextus Empiricus (Against the Physicists = M 10.144–48), which at the same time gives an idea of how Epicurean atoms were understood to behave (at least in Sextus’ time, late 2nd century C.E.). Sextus imagined two atoms separated by a distance of nine minima, traveling at the same speed (as atoms must) toward each other; after four temporal minima, the atoms would be one spatial minimum apart. Then what? They cannot meet in the middle of the remaining distance, by the very concept of a minimum. Nor can one cross the interval before the other, without violating the rule of equal velocity. But how, then, can they meet at all? We do not know an Epicurean reply to this conundrum. Perhaps atoms are always an even number of minimal spaces apart from one another. Or else, minima are always bundled in such large quantities that it is meaningless to speak of an odd or even number of minima between atoms; the Stoics, at all events, held that whether the number of stars, for example, is odd or even is absolutely or naturally non-evident (kathapax adela, Sextus Empiricus Outline of Pyrrhonism = PH 2.97, M 8.147; physei adela, M 8.317–18). But Epicurus believed that motion at the atomic level obeyed different laws from those that appear to operate at the level of macroscopic objects (that atomic motion is discontinuous is an example). Perhaps, then, motions over small numbers of minima were in some sense indiscernible. Finally, it is possible that he discounted such a puzzle as a purely mathematical paradox, since it is recorded that he had little interest in mathematics as a separate science from physics and believed it to be irrelevant to the proper study of physics.