Ética, Parte IV, Proposição 25

The endeavour, wherewith everything endeavours to persist in its being, is defined solely by the essence of the thing itself (III. vii.); from this alone, and not from the essence of anything else, it necessarily follows (III. vi.) that everyone endeavours to preserve his being. Moreover, this proposition is plain from IV. xxii. Coroll., for if a man should endeavour to preserve his being for the sake of anything else, the last--named thing would obviously be the basis of virtue, which, by the foregoing corollary, is absurd. Therefore no one, &c. Q.E.D.