The Senses In General

To explain the eye, we must explain the senses.

It is the soul which feels and not the body. When it is diverted by an ecstasy or strong contemplation, the whole body remains without feeling, although there are various objects which touch it.

The external senses do not sense. It is the brain which does.

We see wounds and illnesses which offend only the brain and impedes all the senses, although the rest of the body does not allow it to be animated.

The impressions from objects in the external senses reach the soul in the brain through the intermediary of the nerves.

We see various accidents which, harming nothing but some nerve, remove the feeling from all the parts of the body where this nerve sends these branches without diminishing anything of that of the others.

How does the soul, residing in the brain, receive the impressions of objects through the nerves?

There are 3 things in these nerves:

1. The skins which envelop them

These take their origin from those which envelop the brain. These are like small pipes divided into several branches which spread here and there through all the limbs in the same way as the veins and arteries

2. Their interior substance

This extends in the form of small nets all along these tubes from the brain, from where it takes its origin, to the extremities of the other members where it attaches.

Each of these small pipes have several of these small nets independent of each other.

3. The animal spirits

These are like a very subtle air or wind, which, coming from the chambers or concavities which are in the brain, flows through these same pipes into the muscles.

The nerves give feeling to the limbs, and also move them.

There are sometimes paralyzes which take away the movement without on that account taking away the feeling, sometimes they said that it there were 2 kinds of nerves, one of which served only for the senses, and the other only for the movements.

Sometimes, that the faculty of feeling was in the skins or membranes, and that of moving was in the interior substance of the nerves, which are things very repugnant to experience and to reason.

Whoever has ever been able to notice any nerve which served the movement without also serving some sense?

How, if it were on the skins that feeling depended, could the various impressions of objects by means of these skins reach the brain?

The Animal Spirits

The spirits flow through the nerves into the muscles, and swelling them according to how the brain distributes them.

This causes the movement of all the limbs. It is the little filaments of which the interior substance of these nerves is composed which serve the senses.

These little nets are enclosed in pipes which are always swollen and held open by the spirits which they contain.

They in no way crowd or impede one another, and are extended from the brain to the extremities of all the members which are capable of feeling, so that, if one touches and does move the place of these members where someone of them is attached, one also makes move at the same moment the place of the brain from which it comes, as well as, pulling one of the ends of a cord which is all tense, one makes the other end move at the same instant.

These nets are thus enclosed in pipes, that the spirits always hold a little swollen and half-open, it is easy to hear that even though they were much more slender than those spun by silkworms, and weaker than those of spiders. ed, they would not appear to be able to extend from the head to the furthest members without any risk of breaking, or that the various situations of these members would prevent their movements.

In order to feel, the soul needs to contemplate some images which are sent by objects to the brain, as our philosophers commonly do.

Or at least it is necessary to conceive the nature of these images quite differently than they do: for, especially since they do not consider anything else in them, except that they must have some resemblance to the objects they represent.

It is impossible for them to show us how they can be formed by these objects, and received by the external sense organs, and transmitted by the nerves to the brain.

They had no reason to suppose them, except that, seeing that our thought can easily be excited by a picture to conceive the object which is painted there, it seemed to them that it should be so in the same way. to conceive of those which touch our senses by a few little pictures which form in our head: instead of that we must consider that there are several things other than images which can excite our thoughts, such as, for example, the signs and the lyrics, which in no way resemble the things they mean.

The objects that we feel truly send their images to the interior of our brain.

There are no images which must in every respect resemble the objects which they represent, for otherwise there would be no distinction between the object and its image, but it suffices that they resemble them in a little things.

Often even that their perfection depends on what they do not resemble them so much as they could do, as you see that the intaglios, being made only of a little ink, placed here and there on paper, represent to us forests, cities, men, and even battles and storms, although, of an infinity of diverse qualities that they make us conceive of in these objects, there is none that the only figure whose resemblance they have properly, and even then this is a very imperfect resemblance, since, on a quite flat surface, they represent to us bodies variously raised and sunken, and that even, according to the rules of perspective, often they better represent circles by ovals than by other circles, and squares by lozenges than by other squares, and so with all the other figures; so that often, to be more perfect in quality of images and better represent an object, they must not resemble it.

The images which are formed in our brain gives the soul a means of feeling all the various qualities of the objects to which they are attached.

When the blind man touches some bodies with his stick, these bodies do not send anything him other than to make his stick move differently according to the qualities which are in them.

They move by the same means the nerves of his hand, and then the places of his brain from which these nerves come.

This gives occasion to his soul to feel as many different qualities in these bodies as there are varieties in the movements which are caused by them in his brain.