to prevent circumlocution--to avoid tautology; it is to be understood,
that it is the property of a being that feels, that thinks, that acts,
to labour to its own happiness; in short, that is called _natural_,
which is conformable to the essence of things, or to the laws, which
Nature prescribes to the beings she contains, in the different orders
they occupy, under the various circumstances through which they are
obliged to pass. Thus health is _natural_ to man in a certain state;
disease is _natural_ to him under other circumstances; dissolution, or
if they will, death, is a _natural_ state for a body, deprived of some
of those things, necessary to maintain the existence of the animal, &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.





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