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. Coroll. 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.