other hand bodies. The bodies are reducible to a certain number of solid corpuscles, indivisible, with differing forms, heavy and animated by an essential and spontaneous motion. These are the atoms which by their coming together constitute bodies.
Now it is evident that atoms in taking the place of other atoms, successively occupy in empty space places that are adequate to them, which have exactly the same extension and the same forms as the respective atoms. If at the moment when an atom is motionless in some place we imagine lines drawn following its contours (as when an object is being traced for transferring), is it not clear that if the atom were removed, we should have preserved its effigy, or a sort of silhouette, its geometric form upon a foundation of empty space? We should obtain thus a portion of space, which I will call an empty atom, in contrast with the full atom which was there before.
Now I ask the atomists to explain what distinguishes the full atom from the empty one, what are the characteristics that may be found in one and not in the other. Is it the being extended? No, for the empty atom is extended like the full atom. Is it the having a form? No, for the empty atom has a form as has the full atom and exactly the same form. Is it the being indivisible? No, for it is still more difficult to understand the divisibility of space than of the body. In a word everything which depends on extension is the same in the empty atom as in the full atom. But the empty atom is not a body and contains nothing corporeal; therefore extension is not the essence of bodies and perhaps does not constitute a part of this essence. May we say that it is the motion which distinguishes the full atom from the empty atom? But before beginning to move the atom must have already been something, because that which is nothing in itself can be neither at rest, nor in motion. Motion, therefore, is a dependent and subordinate phenomenon which already presupposes a defined essence. If we examine carefully we will see that what really distinguishes the full atom from the empty atom is its solidity or weight. Neither solidity nor weight, however, are modifications of extension; both come from force. It is accordingly, force and not extension which constitutes the essence of the body.
Turning now to those who, like the Cartesians, are unwill-
