The Nature of Bodies

The object of the human mind is the human body.

We will know how the human mind differs from other things if we know the nature of the human body.

Generally, a body is better than others in doing many actions or receiving many impressions at once in proportion as its mind is better than others in forming many simultaneous perceptions.

The more the actions of the body depend on itself alone, and the fewer other bodies concur with it in action, the more fitted is the mind of which it is the object for distinct comprehension.

This is why:

• one mind is superior over others
• we are confused about our body
• etc

This is explained by my propositions on the nature of bodies.

Simple Bodies

Axiom 1. All bodies are either in motion or at rest.

Axiom 2. Everybody is moved sometimes more slowly, sometimes more quickly.

LEMMA 1. Bodies are distinguished from each other not through substance, but by motion and speed

Proof: The motion and speed of bodies is self-evident.

Bodies not being distinguished by substance, is plain both from 1.5, 1.8, and 1.15, note.

LEMMA 2. All bodies agree in certain respects.

Proof: All bodies have one and the same attribute (2. Def. 1).

LEMMA 3. A body A in motion or at rest is determined to be at motion or rest by another body B, which is determined to be at motion or rest by a third body C, and so on to infinity.

Proof: Bodies are individual things (2. Def. 1), which (Lemma 1) are distinguished each other in terms of motion and rest;

Thus, (1.28) each must necessarily be determined to motion or rest by another individual thing, namely (2.6), by another body, which other body is also (Ax. 1) in motion or at rest.

This body again can only have been set in motion or caused to rest by being determined by a third body to motion or rest.

This third body again by a fourth, and so on to infinity. Q.E.D.

Corollary: Hence it follows, that a body in motion keeps in motion, until it is determined to a state of rest by some other body.

A body at rest remains so, until it is determined to a state of motion by some other body. This is indeed self—evident.

When body A is at rest and do not take into consideration other bodies in motion, I cannot affirm anything concerning body A, except that it is at rest.

If A afterwards moves, this cannot have resulted from its having been at rest.

If, on the other hand, A is moving, we can only say that it is moving.

If A is subsequently found to be at rest, this rest cannot be the result of A’s previous motion, for such motion can only have led to continued motion.

The state of rest therefore must have resulted from an external cause which was not in A that determined A to be in a state of rest.

Axiom 1: All modes, wherein one body is affected by another body, follow simultaneously from the nature of the body affected and the body affecting

So that one and the same body may be moved in different modes, according to the difference in the nature of the bodies moving it.

On the other hand, different bodies may be moved in different modes by one and the same body.

Axiom 2: When a body in motion impinges on another body at rest, which it is unable to move, it recoils.

In order to continue its motion, and the angle made by the line of motion in the recoil and the plane of the body at rest, whereon the moving body has impinged, will be equal to the angle formed by the line of motion of incidence and the same plane.

So far we have been speaking only of the most simple bodies, which are only distinguished one from the other by motion and rest, quickness and slowness.

Compound Bodies

Definition: When any given bodies of the same or different magnitude are compelled by other bodies to remain in contact, or if they be moved at the same or different rates of speed, so that their mutual movements should preserve among themselves a certain fixed relation, such bodies are in union.

Together, they compose one body. This is distinguished from other bodies by this union.

Axiom 3: In proportion as the parts of an individual, or a compound body, are in contact over a greater or less superficies, they will with greater or less difficulty admit of being moved from their position.

Consequently the body will, with greater or less difficulty, be brought to assume another form.

• Those bodies, whose parts are in contact over large surfaces, are called hard.
• Those, whose parts are in contact over small superficies, are called soft.
• Those, whose parts are in motion among one another, are called fluid.

LEMMA 4: If from a body or individual, compounded of several bodies, certain bodies be separated, and if, at the same time, an equal number of other bodies of the same nature take their place, the individual will preserve its nature as before, without any change in its actuality (forma).

Proof: Bodies (Lemma 1) are not distinguished in respect of substance.

hat which constitutes the actuality (formam) of a body consists (by the last Def.) in a union of bodies; but this union, although there is a continual change of bodies, will (by our hypothesis) be maintained;

The individual, therefore, will retain its nature as before, both in respect of substance and in respect of mode. Q.E.D.

LEMMA 5: If the parts composing an individual become greater or less, but in such proportion, that they all preserve the same mutual relations of motion and rest, the individual will still preserve its original nature, and its actuality will not be changed.

Proof: The same as for the last Lemma.

LEMMA 6: If certain bodies composing an individual be compelled to change the motion, which they have in one direction, for motion in another direction, but in such a manner, that they be able to continue their motions and their mutual communication in the same relations as before, the individual will retain its own nature without any change of its actuality.

Proof: This proposition is self—evident.

For the individual is supposed to retain all that, which, in its definition, we spoke of as its actual being.

LEMMA 7: Furthermore, the individual thus composed preserves its nature, whether it be, as a whole, in motion or at rest, whether it be moved in this or that direction.

So long as each part retains its motion, and preserves its communication with other parts as before.

Proof: This proposition is evident from the definition of an individual prefixed to Lemma 4.

Note: We thus see, how a composite individual may be affected in many different ways, and preserve its nature notwithstanding.

Thus, far we have conceived an individual as composed of bodies only distinguished one from the other in respect of motion and rest, speed and slowness; that is, of bodies of the most simple character.

If, however, we now conceive another individual composed of several individuals of diverse natures, we shall find that the number of ways in which it can be affected, without losing its nature, will be greatly multiplied.

Each of its parts would consist of several bodies, and therefore (by Lemma vi.) each part would admit, without change to its nature, of quicker or slower motion, and would consequently be able to transmit its motions more quickly or more slowly to the remaining parts.

If we further conceive a third kind of individuals composed of individuals of this second kind, we shall find that they may be affected in a still greater number of ways without changing their actuality.

We may easily proceed thus to infinity, and conceive the whole of nature as one individual, whose parts, that is, all bodies, vary in infinite ways, without any change in the individual as a whole.

I should feel bound to explain and demonstrate this point at more length, if I were writing a special treatise on body.

But I have already said that such is not my object.

I have only touched on the question, because it enables me to prove easily that which I have in view.

POSTULATES

1. The human body is composed of a number of individual parts, of diverse nature, each one of which is in itself extremely complex.

2. Of the individual parts composing the human body some are fluid, some soft, some hard.

3. The individual parts composing the human body, and consequently the human body itself, are affected in a variety of ways by external bodies.

4. The human body stands in need for its preservation of a number of other bodies, by which it is continually, so to speak, regenerated.

5. When the fluid part of the human body is determined by an external body to impinge often on another soft part, it changes the surface of the latter, and, as it were, leaves the impression thereupon of the external body which impels it.

6. The human body can move external bodies, and arrange them in a variety of ways.