Emotion, which is called a passivity of the soul, is a
confused idea, whereby the mind affirms concerning its
body, or any part thereof, a force for existence (existendi
vis) greater or less than before, and by the presence of
which the mind is determined to think of one thing rather
than another.
^^^^^Explanation--I say, first, that emotion or passion
of the soul is "a confused idea." For we have shown that
the mind is only passive, in so far as it has inadequate or
confused ideas. (III. iii.) I say, further, "whereby the mind
affirms concerning its body or any part thereof a force for
existence greater than before." For all the ideas of bodies,
which we possess, denote rather the actual disposition of
our own body (II. xvi. Cor. ii.) than the nature of an
external body. But the idea which constitutes the reality
of an emotion must denote or express the disposition of
the body, or of some part thereof, because its power of
action or force for existence is increased or diminished,
helped or hindered. But it must be noted that, when I
say "a greater or less force for existence than before," I
do not mean that the mind compares the present with
the past disposition of the body, but that the idea which
constitutes the reality of an emotion affirms something of
the body, which, in fact, involves more or less of reality
than before.
And inasmuch as the essence of mind consists in the fact
(II. xi., xiii.), that it affirms the actual existence of its own
body, and inasmuch as we understand by perfection the
very essence of a thing, it follows that the mind passes to
greater or less perfection, when it happens to affirm
concerning its own body, or any part thereof, something
involving more or less reality than before.
When, therefore, I said above that the power of the mind
is increased or diminished, I merely meant that the mind
had formed of its own body, or of some part thereof, an
idea involving more or less of reality, than it had already
affirmed concerning its own body. For the excellence of
ideas, and the actual power of thinking are measured by
the excellence of the object. Lastly, I have added "by the
presence of which the mind is determined to think of one
thing rather than another," so that, besides the nature of
pleasure and pain, which the first part of the definition
explains, I might also express the nature of desire.