Ethics, Part III, Proposition 58
Besides pleasure and desire, which are passivities or passions, there are other emotions derived from pleasure and desire, which are attributable to us in so far as we are active.
Proof
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.
Benedict de Spinoza, The Ethics (Ethica Ordine Geometrico Demonstrata), translated from the Latin by R. H. M. Elwes.
First published in The Chief Works of Benedict de Spinoza, vol. II (London: George Bell and Sons, 1884).
This translation is out of copyright in the United States, where it was published in 1884, and wherever the copyright term is the author's life plus 100 years or less; its translator died in 1892.
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Digital text: Project Gutenberg eBook #3800