1. A body, in virtue of its impenetrability, resists the motive force of another body attempting to penetrate the space which it occupies. In spite of the motive force of the second body, the impenetrability of the first body is nonetheless a ground of that second body's rest. It follows from what has already been said that impenetrability just as much presupposes a true force in the parts of me body, in virtue of which they collectively occupy a space, as does the force in virtue of which another body strives to enter this space.

By way of elucidation, imagine two springs which are opposed to each other. Without any doubt they maintain each other in a state of rest by means of forces which are equal to each other. Place between the two a spring of the same elasticity; this spring, by its effort, will produce the same effect, and, in accordance with the rule of the equality of action and reaction, it will maintain the two springs in a state of rest. On the other hand, if this spring is replaced by a solid body which is put between the other two springs, the same effect will be produced: the impenetrability of the solid body will maintain the aforementioned springs in a state of rest. The cause of impenetrability is thus a true force, for it produces the same effect as a true force. Now, if you call attraction a cause, of any kind you please, in virtue of which one body constrains other bodies to press upon the space which it itself occupies or to set it in motion (though here it is sufficient simply to think of this attraction), then impenetrability is a negative attraction. This serves to show that impenetrability is as much a positive ground as any other motive force in nature.