But it occurred to him that upon this hypothesis there could never occur any collisions or combinations of the atoms--nothing but continued and unchangeable parallel lines.

Accordingly, he modified it by saying that the line of descent was not exactly rectilinear, but that each atom deflected a little from the straight line, and each in its own direction and degree; so that it became possible to assume collisions, resiliences, adhesions, combinations, among them, as it had been possible under the variety of original movements ascribed to them by Democritus.

The opponents of Epicurus derided this auxiliary hypothesis; they affirmed that he invented the individual deflection of each atom, without assigning any cause, and only because he was perplexed by the mystery of man's _free-will_.

But Epicurus was not more open to attack on this ground than other physical philosophers.

Most of them (except perhaps the most consistent of the Stoic fatalists) believed that some among the phenomena of the universe occurred in regular and predictable sequence, while others were essentially irregular and unpredictable; each philosopher devised his hypothesis, and recognized some fundamental principle, to explain the first class of phenomena as well as the second.