CHAP. II.
Of Motion, and its Origin.
Motion is an effect by which a body either changes, or has a tendency to change, its position: that is to say, by which it successively corresponds with different parts of space, or changes its relative distance to other bodies. It is motion alone that establishes the relation between our senses and exterior or interior beings: it is only by motion that these beings are impressed upon us—that we know their existence—that we judge of their properties—that we distinguish the one from the other—that we distribute them into classes.
The beings, the substances, or the various bodies of which Nature is the assemblage, are themselves effects of certain combinations or causes which become causes in their turn. A CAUSE is a being which puts another in motion, or which produces some change in it. The EFFECT is the change produced in one body, by the motion or presence of another.
Each being, by its essence, by its peculiar nature, has the faculty of producing, is capable of receiving, has the power of communicating, a variety of motion. Thus some beings are proper to strike our organs; these organs are competent to receiving the impression, are adequate to undergoing changes by their presence. Those which cannot act on any of our organs, either immediately and by themselves, or immediately by the intervention of other bodies, exist not for us; since they can neither move us, nor consequently furnish us with ideas: they can neither be known to us, nor of course be judged of by us. To know an object, is to have felt it; to feel it, it is requisite to have been moved by it. To see, is to have been moved, by something acting on the visual organs; to hear, is to have been struck, by something on our auditory nerves. In short, in whatever mode a body may act upon us, whatever impulse we may receive from it, we can have no other knowledge of it than by the change it produces in us.
Nature, as we have already said, is the assemblage of all the beings, consequently of all the motion of which we have a knowledge, as well as of many others of which we know nothing, because they have not yet become accessible to our senses. From the continual action and re-action of these beings, result a series of causes and effects; or a chain of motion guided by the constant and invariable laws peculiar to each being; which are necessary or inherent to its particular nature—which make it always act or move after a determinate manner. The different principles of this motion are unknown to us, because we are in many instances, if not in all, ignorant of what constitutes the essence of beings. The elements of bodies escape our senses; we know them only in the mass: we are neither acquainted with their intimate combination, nor the proportion of these combinations; from whence must necessarily result their mode of action, their impulse, or their different effects.
Our senses bring us generally acquainted with two sorts of motion in the beings that surround us: the one is the motion of the mass, by which an entire body is transferred from one place to another. Of the motion of this genus we are perfectly sensible.—Thus, we see a stone fall, a ball roll, an arm move, or change its position. The other is an internal or concealed motion, which always depends on the peculiar energies of a body: that is to say, on its essence, or the combination, the action, and re-action of the minute—of the insensible particles of matter, of which that body is composed. This motion we do not see; we know it only by the alteration or change, which after some time we discover in these bodies or mixtures. Of this genus is that concealed motion which fermentation produces in the particles that compose flour, which, however scattered, however separated, unite, and form that mass which we call BREAD. Such also is the imperceptible motion by which we see a plant or animal enlarge, strengthen, undergo changes, and acquire new qualities, without our eyes being competent to follow its progression, or to perceive the causes which have produced these effects. Such also is the internal motion that takes place in man, which is called his INTELLECTUAL FACULTIES, his THOUGHTS, his PASSIONS, his will. Of these we have no other mode of judging, than by their action; that is, by those sensible effects which either accompany or follow them. Thus, when we see a man run away, we judge him to be interiorly actuated by the passion of fear.
Motion, whether visible or concealed, is styled ACQUIRED, when it is impressed on one body by another; either by a cause to which we are a stranger, or by an exterior agent which our senses enable us to discover. Thus we call that acquired motion, which the wind gives to the sails of a ship. That motion which is excited in a body, that contains within itself the causes of those changes we see it undergo, is called SPONTANEOUS. Then it is said, this body acts or moves by its own peculiar energies. Of this kind is the motion of the man who walks, who talks, who thinks. Nevertheless, if we examine the matter a little closer, we shall be convinced, that, strictly speaking, there is no such thing as spontaneous motion in any of the various bodies of Nature; seeing they are perpetually acting one upon the other; that all their changes are to be attributed to the causes, either visible or concealed, by which they are moved. The will of man is secretly moved or determined by some exterior cause that produces a change in him: we believe he moves of himself, because we neither see the cause that determined him, the mode in which it acted, nor the organ that it put in motion.
That is called SIMPLE MOTION, which is excited in a body by a single cause. COMPOUND MOTION, that which is produced by two or more different causes; whether these causes are equal or unequal, conspiring differently, acting together or in succession, known or unknown.
Let the motion of beings be of whatsoever nature it may, it is always the necessary consequence of their essence, or of the properties which compose them, and of those causes of which they experience the action. Each being can only move and act after a particular manner; that is to say, conformably to those laws which result from its peculiar essence, its particular combination, its individual nature: in short, from its specific energies, and those of the bodies from which it receives an impulse. It is this that constitutes the invariable laws of motion: I say invariable, because they can never change, without producing confusion in the essence of things. It is thus that a heavy body must necessarily fall, if it meets with no obstacle sufficient to arrest its descent; that a sensible body must naturally seek pleasure, and avoid pain; that fire must necessarily burn, and diffuse light.