P21: He who imagines what he loves to be affected with Joy or Sadness will also be affected with Joy or Sadness; and each of those affects will be greater or lesser in the lover as they are greater or lesser in the thing loved.

Dem.: The images of things (as we have demonstrated in P19) which posit the existence of a thing loved aid the striving by which the Mind strives to imagine the thing loved. But Joy posits the existence of the joyous thing, and posits more existence, the greater the affect of Joy is. For (by P11 S) it is a transition to a greater perfection. Therefore, the image in the lover of the loved thing’s Joy aids his Mind’s striving, i.e. (by P11 S), affects the lover with Joy, and the more so, the greater this affect was in the thing loved. This was the first thing to be proved.

Next, insofar as a thing is affected with Sadness, it is destroyed, and the more so, the greater the Sadness with which it is affected (by P11 S). So (by P19) he who imagines what he loves to be affected with Sadness, will also be affected with Sadness, and the more so, the greater this affect was in the thing loved, q.e.d.