The Nature of Force

Leibniz confuses impulse with motion. According to Newton, impulse is actually the same as inertial force.

Borelli asserts that impulse is nothing other than a degree of velocity. Some distinguish impulse from effort (conatus), others do not.

Most take motive force to be proportional to motion.

Yet some think there is another kind of force besides the motive one, measured differently—such as by the square of the velocity multiplied by the mass.

But it would be endless to go into all this.

17. Terms like force, gravity, attraction, and the like are useful for reasoning and calculating about motion and moving bodies.

But they are not useful for:

• understanding the simple nature of motion itself, or
• indicating so many distinct qualities.

Newton clearly employed attraction not as a real, physical quality, but merely as a mathematical hypothesis.

Even Leibniz distinguished elementary effort or solicitation from impulse. He admits these entities do not exist in reality, but must be formed by abstraction.

18. The same applies to the composition and resolution of any direct forces into oblique ones using the diagonal and sides of a parallelogram.

These serve mechanics and calculation.

But serving mathematical demonstration is different from describing the nature of things.

19. Many moderns believe that motion is neither destroyed nor created anew, but that the same total quantity of motion always remains.

Aristotle also asked whether motion is generated and destroyed, or eternal (Physics, Book 8).

It is evident to the senses that sensible motion perishes.

Yet they seem to want to assert that the same impulse, effort, or total force remains.

Hence Borelli affirms that the force in impact is not diminished, but expanded; that even contrary impulses can be received and retained in the same body.

Likewise, Leibniz claims that effort exists everywhere and always in matter, and is known by reason where it is not seen by the senses.

But such views are too abstract and obscure—essentially of the same kind as substantial forms and entelechies.

20. All who attempt to explain the cause and origin of motion by a hylarchic principle, or by nature’s deficiency, appetite, or even natural instinct, are speaking rather than thinking.

Nor are those much better who suppose that parts of the earth move themselves, or even that spirits are implanted in them like forms, to account for the acceleration of falling bodies; or who claim that, besides solid extension, something else must be posited in a body to explain its force.

For such people either declare nothing particular or definite—or, if they do, it will be as hard to explain as the very thing it was meant to explain.