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.