E4P42D
Demonstration — Part IV
Latin
Hilaritas (vide ejus definitionem in scholio propositionis 11 partis III) est lætitia quæ quatenus ad corpus refertur, in hoc consistit quod corporis omnes partes pariter sint affectæ hoc est (per propositionem 11 partis III) quod corporis agendi potentia augetur vel juvatur ita ut omnes ejus partes eandem ad invicem motus et quietis rationem obtineant atque adeo (per propositionem 39 hujus) hilaritas semper est bona nec excessum habere potest. At melancholia (cujus etiam definitionem vide in scholio propositionis 11 partis III) est tristitia quæ quatenus ad corpus refertur, in hoc solo consistit quod corporis agendi potentia absolute minuitur vel coercetur adeoque (per propositionem 38 hujus) semper est mala. Q.E.D.
English (Elwes 1883)
Mirth (see its Def. in III. xi. note) is pleasure, which, in so far as it is referred to the body, consists in all parts of the body being affected equally: that is (III. xi.), the body's power of activity is increased or aided in such a manner, that the several parts maintain their former proportion of motion and rest; therefore Mirth is always good (IV. xxxix.), and cannot be excessive. But Melancholy (see its Def. in the same note to III. xi.) is pain, which, in so far as it is referred to the body, consists in the absolute decrease or hindrance of the body's power of activity; therefore (IV. xxxviii.) it is always bad. Q.E.D.
Modern English
Cheerfulness (see its definition in the Scholium of E3P11) is joy that, insofar as it is referred to the body, consists in all parts of the body being equally affected, that is, (E3P11), the body's power of action is increased or aided in such a way that all its parts maintain the same proportion of motion and rest relative to one another. For that reason (E4P39) cheerfulness is always good and cannot have excess. Melancholy (see its definition in the same scholium) is sadness that, insofar as it is referred to the body, consists solely in the absolute diminution or restraint of the body's power of action. Therefore (E4P38) it is always bad. Q.E.D.