אתיקה, חלק IV, משפט 19

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.