injure or be of
service to the mind will be explained in the Fifth Part. But I
would here remark that I consider that a body undergoes death,
when the proportion of motion and rest which obtained mutually
among its several parts is changed. For I do not venture to deny
that a human body, while keeping the circulation of the blood and
other properties, wherein the life of a body is thought to
consist, may none the less be changed into another nature totally
different from its own. There is no reason, which compels me to
maintain that a body does not die, unless it becomes a corpse;
nay, experience would seem to point to the opposite conclusion.
It sometimes happens, that a man undergoes such changes, that I
should hardly call him the same. As I have heard tell of a
certain Spanish poet, who had been seized with sickness, and
though he recovered therefrom yet remained so oblivious of his
past life, that he would not believe the plays and tragedies he
had written to be his own: indeed, he might have been taken for
a grown--up child, if he had also forgotten his native tongue. If
this instance seems incredible, what shall we say of infants? A
man of ripe age deems their nature so unlike his own, that he can
only be persuaded that he too has been an infant by the analogy
of other men. However, I prefer to leave such questions
undiscussed, lest I should give ground to the superstitious for
raising new issues.

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