Action and Reaction

21. Things that cannot be perceived by the senses or understood by reason are of no use in clarifying the nature of things.

We must look to what sense, experience, and reason based on these can tell us.

There are 2 chief kinds of things:

1. Body
2. Mind

We know:

• the extended, solid, mobile, shaped, and sensible body through the senses
• the feeling, perceiving, understanding, and conscious mind by internal awareness.

These things are plainly different and fundamentally heterogeneous. I speak, of course, of known things—for to discuss unknowns is useless.

22. Everything we know and call “body” contains nothing within it that could be the principle or efficient cause of motion.

Impenetrability, extension, and shape neither include nor imply any power to produce motion; indeed, quite the opposite.

These qualities of body are all truly passive. They contain nothing active that could be understood as a source or principle of motion.

“Gravity” really means the physical effect.

When we call a body “heavy”, we mean only that it moves downward, without thinking about the cause.

23. So I boldly declare that body, as a kind of matter, is not the principle of motion.

If anyone insists that by the word “body” he includes, beyond solid extension and its modifications, some occult quality, power, form, or essence, then he may dispute without ideas and misuse names that express nothing distinctly.

A sounder method of philosophizing seems to avoid, as much as possible, abstract and general notions—if they can even be called notions when they cannot be understood.

24. We know everything contained in the idea of body.

What we do know of body is not the principle of motion.

Those who imagine something unknown in the body, of which they have no idea, and call it the principle of motion, are really only saying that the principle of motion is unknown. But it’s tedious to linger too long on such subtleties.

25. Beyond corporeal things, there is another category: thinking beings. From our own experience, we know that these possess the power to move bodies.

For our soul can, at will, initiate and stop the movement of limbs—however it may do so.

Bodies move at the command of the soul.

Therefore, the soul is a principle of motion—though a particular and subordinate one, itself dependent on a primary and universal Principle.

26. Heavy bodies fall downward even when no apparent impulse sets them in motion.

However, one must not conclude from this that the principle of motion lies within them.

Aristotle explains: heavy and light things do not move themselves—for that would imply life, and the power to stop.

All heavy objects follow the same fixed law toward the center of the Earth, and they display no principle or faculty by which they might stop, slow down, increase (except proportionally), or change this motion.

Thus, they are entirely passive.

The same must be said of bodies that cause impact.

Whether moving or in the moment of impact, these bodies are just as passive as when they are at rest.

An inert, resting body acts no more than a moving one, as Newton acknowledges when he states that inertia is the same as momentum.

But an inert and resting body does nothing—therefore neither does the moving one.

27. In truth, a body persists equally in either state—motion or rest.

But this persistence is no more an action of the body than its mere existence could be called an action.

Persistence is just a continuation of the same mode of existence and cannot properly be called action.

The resistance we feel when stopping a moving body is mistakenly attributed to the body’s action.

In fact, that resistance is a sensation within us—it is we who suffer, not the body that acts.

Clearly, we would experience the same thing whether the body moved itself or was moved by some external principle.

28. We speak of action and reaction in bodies, and this is convenient for mechanical demonstrations.

But we must be careful not to suppose from this that there is any real force in them that serves as a cause or principle of motion.

These terms should be understood in the same way as “attraction”—which Newton used not as a real physical quality, but merely as a mathematical hypothesis.

In the same way, the laws of motion and the theorems derived from them remain valid in mechanical philosophy, whether we suppose the effects are caused by the action of bodies attracting one another or by the action of some separate agent moving and governing the bodies.

Similarly, all rules and laws of motion, and the theorems based on them, hold firm as long as the observable effects are accepted and the reasoning is based on them—regardless of whether the cause of these effects lies in the body or in some incorporeal agent.

29. Take away from the idea of body its extension, solidity, and shape—and nothing remains.

But these qualities are indifferent to motion and contain nothing that could be called a principle of motion. This is clear from our very ideas.

So, if by “body” we mean what we conceive, then no principle of motion can be found in it: no part or attribute of it is a true efficient cause that produces motion. To speak a word without conceiving an idea is unworthy of a philosopher.