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