Ethics, Part IV, Proposition 19
Every man, by the laws of his nature, necessarily desires or shrinks from that which he deems to be good or bad.
Proof
The knowledge of good and evil is (IV. viii.) the emotion of pleasure or pain, in so far as we are conscious thereof; therefore, every man necessarily desires what he thinks good, and shrinks from what he thinks bad. Now this appetite is nothing else but man's nature or essence (Cf. the Definition of Appetite, III. ix. note, and Def. of the Emotions, i.). Therefore, every man, solely by the laws of his nature, desires the one, and shrinks from the other, &c. 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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