E3P19D
Demonstration — Part III
Latin
Mens quantum potest ea imaginari conatur quæ corporis agendi potentiam augent vel juvant (per propositionem 12 hujus) hoc est (per scholium propositionis 13 hujus) ea quæ amat. At imaginatio ab iis juvatur quæ rei existentiam ponunt et contra coercetur iis quæ rei existentiam secludunt (per propositionem 17 partis II); ergo rerum imagines quæ rei existentiam ponunt, mentis conatum quo rem amatam imaginari conatur, juvant hoc est (per scholium propositionis 11 hujus) lætitia mentem afficiunt et quæ contra rei amatæ existentiam secludunt, eundem mentis conatum coercent hoc est (per idem scholium) tristitia mentem afficiunt. Qui itaque id quod amat destrui imaginatur, contristabitur, etc. Q.E.D.
English (Elwes 1883)
The mind, as far as possible, endeavours to conceive those things which increase or help the body's power of activity (III. xii.); in other words (III. xii. note), 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 (II. xvii.); 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 (III. xi. note), 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.
Modern English
The mind strives as far as it can to imagine those things that increase or assist the body's power of acting (E3P12), that is (E3P13S), those things it loves. Imagination is helped by what posits the existence of a thing and is checked by what excludes it (E2P17). The images of things that posit the existence of what is loved therefore help the mind's striving to imagine the loved thing, that is (E3P11S), they affect the mind with joy. And the images that exclude the existence of the loved thing check that same striving of the mind, that is (E3P11S), they affect the mind with sadness. Whoever therefore conceives that what they love is destroyed will feel sadness, etc. Q.E.D.