PROP. IV. Nothing can be destroyed, except by a cause external to itself.

Proof.—This proposition is self—evident, for the definition of anything affirms the essence of that thing, but does not negative it; in other words, it postulates the essence of the thing, but does not take it away. So long therefore as we regard only the thing itself, without taking into account external causes, we shall not be able to find in it anything which could destroy it. Q.E.D.