In the previous chapters I have shown that in Newton's view, force was a substance, and in Leibniz's view analogous to it. This means that the underlying idea about the action of force was that force is the cause of changes in motion and is simultaneously the change itself, which is, as it were, transferred. It would, however, be wrong to consider this double meaning as vagueness or ambiguity, since these are two aspects of one and the same thing. It is more appropriate to call the concept of force a unity-in-duality. To clarify this, I have used the metaphor of water pouring from one barrel to another. This metaphor justifies my calling this a remnant of the substantial concept of force in scholastics. After all, when scholasticism turned to the concept of impetus for a solution to the problem of motion, the cause of the perseverance of motion was equated to the cause of the change in motion.