The Physics of Epicurus were borrowed in the main from the atomic theory of Democritus, but were modified by him in a manner subservient and contributory to his ethical scheme. To that scheme it was essential that those celestial, atmospheric, or terrestrial phenomena that the public around him ascribed to the agency and purposes of the gods, should be understood as being produced by physical causes. An eclipse, an earthquake, a storm, a shipwreck, unusual rain or drought, a good or a bad harvest--and not merely these, but many other occurrences far smaller and more unimportant, as we may see by the eighteenth chapter of the Characters of Theophrastus--were then regarded as visitations of the gods, requiring to be interpreted by recognized prophets, and to be appeased by ceremonial expiations. When once a man became convinced that all these phenomena proceeded from physical agencies, a host of terrors and anxieties would disappear from the mind; and this Epicurus asserted to be the beneficent effect and real recommendation of physical philosophy. He took little or no thought for scientific curiosity as a motive _per se_, which both Democritus and Aristotle put so much in the foreground.
Epicurus adopted the atomistic scheme of Democritus, but with some important variations. He conceived that the atoms all moved with equal velocity in the downward direction of gravity. 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. Plato admitted an invincible Erratic necessity; Aristotle introduced Chance and Spontaneity; Democritus multiplied indefinitely the varieties of atomic movements. The hypothetical deflexion alleged by Epicurus was his way, not more unwarranted than the others, of providing a fundamental principle for the unpredictable phenomena of the universe. Among these are the mental (including the volitional) manifestations of men and animals; but there are many others besides; and there is no ground for believing that the mystery of free-will was peculiarly present to his mind. The movements of a man or animal are not exclusively subject to gravitation and other general laws; they are partly governed by mental impulses and by forces of the organism, intrinsic and peculiar to himself, unseen and unfelt by others. For these, in common with many other untraceable phenomena in the material world, Epicurus provides a principle in the supplementary hypothesis of deflexion. He rejected the fatalism contained in the theories of some of the Stoics, and admitted a limited range of empire to chance, or irregularity. But he maintained that the will, far from being among the phenomena essentially irregular, is under the influence of motives; for no man can insist more strenuously than he does (see the Letter to Menoecens) on the complete power of philosophy,--if the student could be made to feel its necessity and desire the attainment of it, so as to meditate and engrain within himself sound views about the gods, death, and human life generally,--to mould our volitions and character in a manner conformable to the exigencies of virtue and happiness.