3p30d.md (523B)
1 He who conceives, that he affects others with pleasure 2 or pain, will, by that very fact, himself be affected with 3 pleasure or pain (III. xxvii.), but, as a man (II. xix. and 4 xxiii.) is conscious of himself through the modifications whereby 5 he is determined to action, it follows that he who conceives, 6 that he affects others pleasurably, will be affected with 7 pleasure accompanied by the idea of himself as cause; in other 8 words, he will regard himself with pleasure. And so mutatis 9 mutandis in the case of pain. Q.E.D. 10 11