E3P30D
Demonstration — Part III
Latin
Qui se reliquos lætitia vel tristitia afficere imaginatur, eo ipso (per propositionem 27 hujus) lætitia vel tristitia afficietur. Cum autem homo (per propositiones 19 et 23 partis II) sui sit conscius per affectiones quibus ad agendum determinatur, ergo qui aliquid egit quod ipse imaginatur reliquos lætitia afficere, lætitia cum conscientia sui tanquam causa afficietur sive seipsum cum lætitia contemplabitur et contra. Q.E.D.
English (Elwes 1883)
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.
Modern English
Whoever imagines that he affects others with joy or sadness will by that very fact be affected with joy or sadness (E3P27). A person is conscious of himself through the affections by which he is determined to act (E2P19). Therefore, whoever does something he imagines to affect others with joy will be affected with joy accompanied by consciousness of himself as cause, that is, he will regard himself with joy. And conversely. Q.E.D.