Prop. XL. Whatsoever conduces to man's social life, or causes men to live together in harmony, is useful, whereas whatsoever brings discord into a State is bad.

Proof.—For whatsoever causes men to live together in harmony also causes them to live according to reason (IV. xxxv.), and is therefore (IV. xxvi. xxvii.) good, and (for the same reason) whatsoever brings about discord is bad. Q.E.D.

Prop. XLI. Pleasure in itself is not bad but good : contrariwise, pain in itself is bad.

Proof.—Pleasure (III. xi. and note) is emotion, whereby the body's power of activity is increased or helped ; pain is emotion, whereby the body's power of activity is diminished or checked ; therefore (IV. xxxviii.) pleasure in itself is good, &c. Q.E.D.

Prop. XLII. 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.

Prop. XLIII. Stimulation may be excessive and bad ; on the other hand, grief may be good, in so far as stimulation or pleasure is bad.

Proof.—Localized pleasure or stimulation (titillatio) is pleasure, which, in so far as it is referred to the body, consists in one or some of its parts being affected more than the rest (see its Definition, III. xi. note) ; the power of this emotion may be sufficient to overcome other actions of the body (IV. vi.), and may remain obstinately fixed therein, thus rendering it incapable of being affected in a variety of other ways : therefore (IV. xxxviii.) it may be bad. Again, grief, which is pain, cannot as such be good (IV. xli.). But, as its force and increase is defined by the power of an external cause compared with our own (IV. v.), we can conceive infinite degrees and modes of strength in this emotion (IV. iii.) ; we can, therefore, conceive it as capable of restraining stimulation, and preventing its becoming excessive, and hindering the body's capabilities ; thus, to this extent, it will be good. Q.E.D.

Prop. XLIV. Love and desire may be excessive.

Proof.—Love is pleasure, accompanied by the idea of an external cause (Def. of Emotions, vi.) ; therefore stimulation, accompanied by the idea of an external cause is love (III. xi. note) ; hence love maybe excessive. Again, the strength of desire varies in proportion to the emotion from which it arises (III. xxxvii.). Now emotion may overcome all the rest of men's actions (IV. vi.) ; so, therefore, can desire, which arises from the same emotion, overcome all other desires, and become excessive, as we showed in the last proposition concerning stimulation.