But in so far as the mind (III.

i.) has inadequate ideas, it is necessarily passive: wherefore the activities of the mind follow solely from adequate ideas, and accordingly the mind is only passive in so far as it has inadequate ideas.

Q.E.D.
Note .-- Thus we see, that passive states are not attributed to the mind, except in so far as it contains something involving negation, or in so far as it is regarded as a part of nature, which cannot be clearly and distinctly perceived through itself without other parts: I could thus show, that passive states are attributed to individual things in the same way that they are attributed to the mind, and that they cannot otherwise be perceived, but my purpose is solely to treat of the human mind.
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.