The Sensible Bodies and Insensible Particles

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 the sensation of:

• titillation and pain
• 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 are not out of our eye.

When we stop our ear with our finger we hear a humming sound from the agitation of the air that is shut up within it.

The following are produced in bodies by the motion of certain other bodies:

• heat [hardness, weight]
• the other sensible qualities in objects
• the forms of those bodies that are purely material such as the forms of fire

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. This treatise has explained all the phenomenon of nature

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 uses principles which are 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.