PROPOSITION IX. DEFINITION. Contact is the reciprocal application of the forces of impenetrability of several elements.

Scholium. Contact is commonly defined in terms of immediate presence. But even if you insisted on adding external (for without this addition, God, who is immediately present to all things, albeit internally present, would have to be thought of as touching them), the definition will hardly be thought to be complete in all respects. For since others have succeeded in satisfactorily establishing that bodies separated by empty space can nevertheless coexist and can therefore be immediately present to each other, though without reciprocal contact, this definition will doubtless be found to have its weaknesses. The Newtonian School, not without a great show of truth, defended the immediate attraction of bodies, even at a distance from each other, their co-presence, nonetheless, occurring in the absence of reciprocal contact. Furthermore, if one defends the definition which attempts to substitute immediate co-presence for the notion of contact itself, one ought first to explain the notion of this presence. If, as is usual, it is explained in terms of reciprocal action, in what, I ask, does the action consist? Undoubtedly, bodies act by moving each other. But the moving force, which is exerted from a given point, either repels other bodies from that point, or attracts them to it. It is obvious which of the two actions is to be understood as involved in contact. For, in moving one body closer and closer to another, we say that they are touching each other when the force of impenetrability, that is to say, of repulsion, is felt. Thus, the action and reaction of the different elements against each other constitutes the genuine notion of contact.

PROPOSITION X. THEOREM. Bodies would not have a determinate volume in virtue of the force of impenetrability alone; there must be another force, which is likewise inherent in them, the force of attraction. The two forces together define the limit of the extension of bodies.

The force of impenetrability is a repulsive force, which prevents anything external from approaching more closely.