Page:The System of Nature.djvu/35

 animal, &amp;c. By ESSENCE is to be understood, that which constitutes a being, such as it is; the whole of the properties or qualities by which it acts as it does. Thus, when it is said, it is the essence of a stone to fall, it is the same as saying that its descent is the necessary effect of its gravity—of its density—of the cohesion of its parts—of the elements of which it is composed. In short, the essence of a being is its particular, its individual nature.

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.