xvi.) must necessarily be defined by the power of an external cause compared with our own.

Q.E.D.
PROP.VI.

The force of any passion or emotion can overcome the rest of a man's activities or power, so that the emotion becomes obstinately fixed to him.
Proof .-- The force and increase of any passion and its persistence in existing are defined by the power of an external cause compared with our own (by the foregoing Prop.); therefore (IV.

iii.) it can overcome a man's power, &e.

Q.E.D.
PROP.VII.