The Sensible Bodies and Insensible Particles

¹⁹⁶ The soul perceives only in so far as it is in the brain

The soul does not perceive in so far as it is in each member of the body. It can only perceive when it is in the brain where the nerves, by their movements, convey to it the actions of the external objects that touch the parts of the body in which they are inserted.

There are 3 proofs for this:

1. There are various maladies which affect the brain alone. Yet bring disorder on, or deprive us of the use of, our senses.

This is the same way that sleep only affects the brain. Yet it takes from us the faculty of perception which afterwards is restored to us in our waking state.

2. The obstruction to the movement of the nerves that extend from the brain to the end of those nerves can take away sensation from the part of the body.

This happens even if there is no disease in the brain [or in the parts where the organs of the external senses are].

3. We sometimes feel pain in certain body parts even if the real cause is somewhere nearer the brain.

A girl suffered from a bad ulcer in the hand. She had her eyes bandaged whenever the surgeon came to dress it as she was unable to bear the sight of the dressing of the sore.

The gangrene spread. After a few days, the arm was amputated from the elbow without the girl’s knowledge.

Linen cloths tied one above the other were substituted in place of the part amputated, so that she remained for some time without knowing that the operation had been performed.

Meanwhile, she complained of feeling various pains, sometimes in one finger of the hand that was cut off, and sometimes in another.

The only explanation is that the nerves which before stretched from the brain to the hand, but now ended in near the elbow, were moved in the same way as they did before the amputation. This movement still impressed on the mind in the brain the sensation of pain in the finger.

This clearly shows that the pain of the hand is not felt by the mind in so far as it is in the hand, but in so far as it is in the brain.

¹⁹⁷ The mind can be excited by the various sensations from the motion of body.

The motions of the body alone are sufficient to excite all sorts of thoughts in the mind, even if those motions do not resemble those thoughts.

Confused thoughts are called sensations (SENSUS, SENSATIONES). These especially can give rise to such thoughts.

Words, whether uttered or merely written, excite in our minds all kinds of thoughts and emotions.

On the same paper, with the same pen and ink, by merely moving the point of the pen over the paper in a particular way, we can trace letters that will raise in the minds of our readers the thoughts of combats, tempests, or the furies, and the passions of indignation and sorrow; in place of which, if the pen be moved in another way hardly different from the former, this slight change will cause thoughts widely different from the above, such as those of repose, peace, pleasantness, and the quite opposite passions of love and joy.

Someone will object that writing and speech do not immediately excite in the mind any passions, or imaginations of things different from the letters and sounds, but afford simply the knowledge of these, on occasion of which the mind, understanding the signification of the words, afterwards excites in itself the imaginations and passions that correspond to the words. But what will be said of the sensations of pain and titillation?

The motion merely of a sword cutting a part of our skin causes pain, [but does not on that account make us aware of the motion or figure of the sword].

It is certain that this sensation of pain is not less different from the motion that causes it, or from that of the part of our body which the sword cuts, than are the sensations we have of colour, sound, odour, or taste. On this ground we may conclude that our mind is of such a nature that the motions alone of certain bodies can also easily excite in it all the other sensations, as the motion of a sword excites in it the sensation of pain.

¹⁹⁸ Our senses only tell us of the shape, location, magnitude, and motion of external objects.

We observe no such difference between the nerves as to lead us to judge that one set of them convey to the brain from the organs of the external senses anything different from another, or that anything at all reaches the brain besides the local motion of the nerves themselves.

Local motion alone causes in us not only the sensation of titillation and of pain, but also of light and sounds.

If we receive a blow on the eye of sufficient force to cause the vibration of the stroke to reach the retina, we see numerous sparks of fire, which, nevertheless, are not out of our eye.

When we stop our ear with our finger, we hear a humming sound, the cause of which can only proceed from the agitation of the air that is shut up within it.

Heat [hardness, weight], and the other sensible qualities in objects, and also the forms of those bodies that are purely material, as, for example, the forms of fire, are produced in them by the motion of certain other bodies. These in their turn likewise produce other motions in other bodies.

The motion of one body may be caused by the motion of another. It can be diversified by the size, figure, and location of its parts. But we are unable to conceive how these same things (viz., size, figure, and motion), can produce something else of a nature entirely different from themselves, as, for example, those substantial forms and real qualities which many philosophers suppose to be in bodies.

We cannot think how these qualities or forms possess force to cause motions in other bodies.

According to the nature of our soul, the diverse motions of body produce in it all the sensations which it has.

Several of its sensations are in reality caused by such motions.

Only these motions pass from the external sense organs to the brain.

The various dispositions of external objects such as light, colour, smell, taste, sound, heat or cold, and the other tactile qualities have the power of moving our nerves in various ways.

¹⁹⁹ There is no phenomenon of nature whose explanation has been omitted in this treatise.

Beyond what is perceived by the senses, there is nothing that can be considered a phenomenon of nature.

But leaving out of account motion, magnitude, figure, [and the situation of the parts of each body], which I have explained as they exist in body, we perceive nothing out of us by our senses except light, colours, smells, tastes, sounds, and the tactile qualities.

These I have recently shown to be nothing more, at least so far as they are known to us, than certain dispositions of the objects, consisting in magnitude, figure, and motion.

²⁰⁰ This treatise contains no principles which are not universally received

This philosophy is not new, but of all others the most ancient and common.

I have here explained the whole nature of material things.

I have nevertheless made use of no principle which was not received and approved by Aristotle, and by the other philosophers of all ages.

My philosophy is not new. It is of all others the most ancient and common.

I have in truth merely considered the shape, motion, and magnitude of bodies, and examined what must follow from their mutual concourse on the principles of mechanics, which are confirmed by certain and daily experience.

But no one ever doubted that bodies are moved, and that they are of various sizes and figures, according to the diversity of which their motions also vary, and that from mutual collision those somewhat greater than others are divided into many smaller, and thus change figure. We have experience of the truth of this, not merely by a single sense, but by several, as touch, sight, and hearing:

We also distinctly imagine and understand it. This cannot be said of any of the other things that fall under our senses, as colours, sounds, and the like; for each of these affects but one of our senses, and merely impresses upon our imagination a confused image of itself, affording our understanding no distinct knowledge of what it is.

²⁰¹ Sensible bodies are composed of insensible particles

There are many particles in each body that are not perceived by our senses. This will not be approved of by people who take the senses as the measure of the knowable.

There are bodies so small that are not perceptible by any of our senses.

A tree increases daily. It is impossible to think how it becomes larger than before, unless we also think at the same time that some particles were added to it.

But whoever observed by the senses those small particles that are added daily to the tree while growing?

Some philosophers think that quantity is indefinitely divisible. They should admit that in the division, the parts may become so small as to be wholly imperceptible.

We are unable to perceive very minute bodies.

This is because, for something to be perceived, it must move the nerves.

But the nerves are like small cords, being composed of smaller fibres. Thus, the most minute bodies are not capable of moving them.

Nor do I think that any reasonable person will not deny that we philosophize with much greater truth when we judge of what happens in those small bodies which are imperceptible from their minuteness only, after the analogy of what we see occurring in those we do perceive,

In this way, we explain all that is in nature.

than when we give an explanation of the same things by inventing new novelties that have no relation to the things we actually perceive.

Examples are first matter, substantial forms, and all that grand array of qualities which philosophers suppose.

Each of these is more difficult to comprehend than all that is to be explained by means of them.