Spinoza's Ethics: III.P21 - Responsiveness to Beloved

Qui id quod amat lætitia vel tristitia affectum imaginatur, lætitia etiam vel tristitia afficietur et uterque hic affectus major aut minor erit in amante prout uterque major aut minor est in re amata.

Whoever imagines that thing which one loves affected by joy or sadness, also is affected by joy or sadness and either of these emotions will be more or less in loving just as there is more or less in the beloved one.

DEMONSTRATIO: Rerum imagines (ut in propositione 19 hujus demonstravimus) quæ rei amatæ existentiam ponunt, mentis conatum quo ipsam rem amatam imaginari conatur, juvant. Sed lætitia existentiam rei lætæ ponit et eo magis quo lætitiæ affectus major est : est enim (per scholium propositionis 11 hujus) transitio ad majorem perfectionem : ergo imago lætitiæ rei amatæ in amante ipsius mentis conatum juvat hoc est (per scholium propositionis 11 hujus) amantem lætitia afficit et eo majore quo major hic affectus in re amata fuerit. Quod erat primum. Deinde quatenus res aliqua tristitia afficitur eatenus destruitur et eo magis quo majore afficitur tristitia (per idem scholium propositionis 11 hujus) adeoque (per propositionem 19 hujus) qui id quod amat tristitia affici imaginatur, tristitia etiam afficietur et eo majore quo major hic affectus in re amata fuerit. Q.E.D.

The images of things (as in IIIP19 we have shown) which confirms the existence of a beloved thing, helps the effort of the mind by which the beloved thing itself is imagined. But joy confirms the existence of the joyous thing and the greater it is the more the emotion of joy: for it is (by IIIP11S) a movement to a greater perfection; therefore the image of joy of the beloved thing helps the effort of the mind itself in loving that is (by IIIP11S) joy affects loving and the more it does the more here the emotion will have been in the beloved thing. This was first. Then insofar as another thing is affected with sadness thus far is it destroyed and the more it does the greater it is affected with sadness (by IIIP11S again) to such a degree (by IIIP19) who imagines what that person loves to be affected with sadness, also will be affected by sadness and the more so the more the emotion will have been here in the beloved thing.