Selection from – Ethics – Part III. On the Nature and Origin of the Emotions (Page 7)

Spinoza's Words:(on pleasure derived from preservation of loved thing)

Proposition. XIX. He who conceives that the object of his love is destroyed will feel pain; if he conceives that it is preserved he will feel pleasure.

Proof.—The mind, as far as possible, endeavours to conceive those things which increase or help the body's power of activity; in other words those things which it loves. But conception is helped by those things which postulate the existence of a thing, and contrariwise is hindered by those which exclude the existence of a thing; therefore the images of things, which postulate the existence of an object of love, help the mind's endeavour to conceive the object of love, in other words, affect the mind pleasurably; contrariwise those things, which exclude the existence of an object of love, hinder the aforesaid mental endeavour; in other words, affect the mind painfully. He, therefore, who conceives that the object of his love is destroyed will feel pain, &c. Q.E.D.

Proposition. XX. He who conceives that the object of his hate is destroyed will also feel pleasure.