Ethics, Part IV, Proposition 42
Mirth cannot be excessive, but is always good; contrariwise, Melancholy is always bad.
Proof
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.
Benedict de Spinoza, The Ethics (Ethica Ordine Geometrico Demonstrata), translated from the Latin by R. H. M. Elwes.
First published in The Chief Works of Benedict de Spinoza, vol. II (London: George Bell and Sons, 1884).
This translation is out of copyright in the United States, where it was published in 1884, and wherever the copyright term is the author's life plus 100 years or less; its translator died in 1892.
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Digital text: Project Gutenberg eBook #3800