Ética, Parte III, Proposição 58

When the mind conceives itself and its power of activity, it feels pleasure (III. liii.): now the mind necessarily contemplates itself, when it conceives a true or adequate idea (II. xliii.). But the mind does conceive certain adequate ideas (II. xl. note 2.). Therefore it feels pleasure in so far as it conceives adequate ideas; that is, in so far as it is active (III. i.). Again, the mind, both in so far as it has clear and distinct ideas, and in so far as it has confused ideas, endeavours to persist in its own being (III. ix.); but by such an endeavour we mean desire (by the note to the same Prop.); therefore, desire is also attributable to us, in so far as we understand, or (III. i.) in so far as we are active. Q.E.D.