“As the atoms are of various shapes, and the collision will not take place in the center of the atoms, then, even according to the principles of modern mechanical science, revolutions of the atoms on their axes and lateral motions will be set up. When once set up, these lateral motions must ever become more and more complicated, and as the collision of constant new atoms with a layer of atoms already in lateral motion constantly imparts new forces, so we may suppose that the motion will continually increase. From the lateral motions in connection with the rotation of the atoms are then easily produced cases of retrogressive movement. If now, in a layer of atoms so involved, the heavier--i. e., the larger--atoms continually receive a stronger impetus downwards, they will finally be collected below, while the light ones will form the upper stratum.”[15] Aristotle said that if there could be such a condition as space devoid of substance (matter), then in such a vacuum all bodies, light or heavy, would fall with equal velocity, which he considered an absurdity. For him the differences in the speed of falling bodies are entirely dependent upon differences in the density of the medium--air or water--through which they fall toward the center of the earth. Epicurus agreed with the Stagirite on the point of equal velocities in a non-resisting medium. But Aristotle not only denied the possibility of the existence of a void anywhere, but asserted that in such a void--were a vacuum possible--no motion could take place! Epicurus believed that all atoms, of whatever weight, would fall with equal velocity in a vacuum, in simple parallel lines. They are in everlasting movement, originally a perpetual, equable falling through the boundless infinity of vacuous space, with an incomparably greater speed than that of light-rays; though he believed that these traverse the distance from sun to earth in an instant.[16] While it is true that in infinite space there is neither an “up” nor a “down,” the fact remains, as Epicurus in effect observed, that man considers his head “up” and his feet “down,” and a body as being relatively distant from, or as near or on or within, the earth. So the atoms do converge at the earth, falling downwards unless met by bodies which, by a particular movement and weight, drive them to one side or upwards, or in a spiral or whirling motion--“the commencement of the formation of worlds.” But if the movement of atoms is in straight parallel lines, and all move in the vacuum with equal velocity, how could one atom drive another “to one side or upwards”?