particular, its individual nature. 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.