The Distinction Between Force and the Quantity of Motion Is Important, Among Other Reasons, as Showing That We Must Have Recourse to Metaphysical Considerations in Addition to Discussions of Extension if We Wish to Explain the Phenomena of Matter

This consideration of force as distinguished from the quantity of motion is of importance, not only in physics and mechanics for finding the real laws of nature and the principles of motion, and even for correcting many practical errors which have crept into the writings of certain able mathematicians, but it is also of importance in metaphysics for the better understanding of principles. Because motion, if we regard only its exact and formal meaning, that is, change of place, is not something entirely real, and when several bodies change places, it is not possible, by considering these changes alone, to determine to which of the bodies movement or repose is to be attributed, as I could demonstrate geometrically, if I wished to stop for it now. But the force or the proximate cause of these changes is something more real, and there are sufficient grounds for attributing it to one body rather than to another, and it is only through this latter investigation that we can determine to which one the movement must appertain. Now his force is something different from size, figure or motion, and it can be seen from this consideration that the concept of a body does not consist solely of extension and the modifications of extension as our moderns persuade themselves. We are therefore obliged to restore certain beings or forms which they have banished. It appears more and more clear that although all the particular phenomena of nature can be explained mathematically or mechanically by those who understand them, yet nevertheless, the general principles of corporeal nature and even of mechanics are metaphysical rather than geometrical and have reference to certain indivisible forms or natures as the causes of the appearances, rather than to the corporeal mass or to extension. This reflection is able to reconcile the mechanical philosophy of the moderns with the circumspection of those intelligent and well-meaning persons who fear, not without reason, that we are straying too far from immaterial beings, to the detriment of piety.