P19: He who imagines that what he loves is destroyed will be saddened; but he who imagines it to be preserved, will rejoice.

Dem.: Insofar as it can, the Mind strives to imagine those things that increase or aid the Body’s power of acting (by P12), i.e. (by P13 S), those it loves. But the imagination is aided by what posits the existence of a thing, and on the other hand, is restrained by what excludes the existence of a thing (by IIP17). Therefore, the images of things that posit the existence of a thing loved aid the Mind’s striving to imagine the thing loved, i.e. (by P11 S), affect the Mind with Joy. On the other hand, those which exclude the existence of a thing loved, restrain the same striving of the Mind, i.e. (by P11 S), affect the Mind with Sadness. Therefore, he who imagines that what he loves is destroyed will be saddened, etc., q.e.d.