P22: If we imagine someone to affect with Joy a thing we love, we shall be affected with Love toward him. If, on the other hand, we imagine him to affect the same thing with Sadness, we shall also be affected with Hate toward him.

Dem.: He who affects a thing we love with Joy or Sadness affects us also with Joy or Sadness, if we imagine that the thing loved is affected by that Joy or Sadness (by P21).

But this Joy or Sadness is supposed to be accompanied in us by the idea of an external cause.

Therefore (by P13 S), if we imagine that someone affects with Joy or Sadness a thing we love, we shall be affected with Love or Hate toward him, q.e.d.

Schol.: P21 explains to us what Pity is, which we can define as Sadness that has arisen from injury to another. By what name we should call the Joy that arises from another’s good I do not know. Next, Love toward him who has done good to another we shall call Favor, and Hatred toward him who has done evil to another we shall call Indignation.

Finally, it should be noted that we do not pity only a thing we have loved (as we have shown in P21), but also one toward which we have previously had no affect, provided that we judge it to be like us (as I shall show below). And so also we favor him who has benefited someone like us, and are indignant at him who has injured one like us.