The Nature of Motion

43. What is the nature of motion?

Since motion is clearly perceived by the senses, it has been obscured not so much by its own nature as by the learned commentaries of philosophers.

Motion never impinges on our senses without bodily mass, space, and time.

However, some people think of motion as a simple and abstract idea, separated from all other things.

• But that most tenuous and subtle idea eludes the keenness of the intellect.

Hence arise great difficulties about the nature of motion, with obscure definitions.

Such are those definitions of Aristotle and the Scholastics.

They say that motion is:

• the act of a movable thing insofar as it is movable, or
• the act of a being in potency insofar as it is in potency.

Such also is a recent idea that nothing in motion is real except that momentary [aspect] which must be constituted in a force striving towards change.

Such people intended to explain the abstract nature of motion, excluding all consideration of time and space.

But there is no way to understand that abstract quintessence of motion.

44. They divide and separate the parts of motion itself from each other.

• They form distinct ideas of them as if they were truly distinct entities.

Some distinguish motion from movement.

• They view movement as an instantaneous element of motion.

They wish velocity, endeavor, force, impetus to be diverse in essence. Each of them is presented to the intellect through its own and abstract idea, separated from all others.

But these are absurd.

45. Many also define motion by transition, forgetting that transition itself:

• cannot be understood without motion
• should be defined by motion.

Those definitions bring light to some things, but also bring darkness to others.

From this zeal for defining and abstracting, many most subtle questions have arisen about motion and other things.

These same questions, being of no use, have tormented the minds of men in vain.

Aristotle himself frequently confesses that motion is difficult to understand.

Mathematics teaches the infinite division of time and space.

• This has introduced paradoxes and thorny theories (such as are all those in which infinity is treated) into speculations about motion.

But whatever is of this kind, it all has motion in common with space and time, or rather, refers its explanation to them.

47. The nature of motion has been rendered perplexing by:

• excessive abstraction or division of truly inseparable things
• the confusion of very different things

It is customary to confuse motion with the efficient cause of motion.

This makes motion two-faced. It has:

• one face obvious to the senses
• the other enveloped in murky night

This obscurity and confusion creates various paradoxes about motion, when the error is simply taking the effect as the cause.

48. This leads to the opinion that the same quantity of motion is always conserved.

This is false.

Aristotle, in Book 8 of the Physics, inquires whether:

• motion has been made and corrupted, or
• it has existed from eternity as an immortal life inherent in all things.

He seems to have understood a vital principle rather than an external effect, or change of place.

49. Many also suspect that motion is not a mere passive state in bodies.

But if we understand that which is presented to the senses in the motion of a body, no one can doubt that it is entirely passive.

For what does the successive existence of a body in different places have in itself that suggests action, or is it anything other than a bare and inert effect?

50. The Peripatetics say that motion is a single act of both, the mover and the moved.

They do not sufficiently distinguish the cause from the effect.

Similarly, those who imagine an endeavor or striving in motion, or think that the same body is simultaneously carried in opposite directions, seem to be deluded by the same confusion of ideas, the same ambiguity of words.

It is a primary law of nature that a body perseveres equally in the state of motion as of rest, as long as nothing happens from elsewhere to change that state.

Therefore it is gathered that the force of inertia, under different respects, is either resistance or impetus.

In this sense, a body is indifferent by its nature to motion or rest.

• It is as difficult to induce rest in a moving body as motion in a resting one.

But since a body equally preserves either state, why should it not be indifferent to either?