On the other hand, if he has done anything which he conceives as affecting others painfully, he will regard himself with pain.
Proof .-- He who conceives, that he affects others with pleasure or pain, will, by that very fact, himself be affected with pleasure or pain (III.

xxvii.), but, as a man (II.xix.

and xxiii.) is conscious of himself through the modifications whereby he is determined to action, it follows that he who conceives, that he affects others pleasurably, will be affected with pleasure accompanied by the idea of himself as cause; in other words, he will regard himself with pleasure.

And so mutatis mutandis in the case of pain.

Q.E.D.
Note .-- As love (III.