Éthique, Partie IV, Proposition 42

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.