An Affect that is called a Passion of the mind is a confused idea, by which the Mind affirms of its Body, or of some part of it, a greater or lesser force of existing than before, which, when it is given, determines the Mind to think of this rather than that.

Exp.: I say, first, that an Affect, or passion of the mind, is a confused idea. For we have shown (P3) that the Mind is acted on only insofar as it has inadequate, or confused, ideas.

Next, I say by which the mind affirms of its body or of some part of it a greater or lesser force of existing than before. For all the ideas that we have of bodies indicate the actual constitution of our own Body (by IIP16 C2) more than the nature of the external body. But this {idea}, which constitutes the form of the affect, must indicate or express a constitution of the Body (or of some part of it), which the Body (or some part of it) has because its power of acting, or force of existing, is increased or diminished, aided or restrained.

But it should be noted that, when I say a greater or lesser force of existing than before, I do not understand that the Mind compares its Body’s present constitution with a past constitution, but that the idea which constitutes the form of the affect affirms of the body something which really involves more or less of reality than before.

And because the essence of the Mind consists in this (by IIP11 and P13), that it affirms the actual existence of its body, and we understand by perfection the very essence of the thing, it follows that the Mind passes to a greater or lesser perfection when it happens that it affirms of its body (or of some part of the body) something which involves more or less reality than before. So when I said above that the Mind’s power of Thinking is increased or diminished, I meant nothing but that the Mind has formed of its Body (or of some part of it) an idea which expresses more or less reality than it had affirmed of the Body.

Finally, I added which determines the Mind to think of this rather than that in order to express also, in addition to the nature of Joy and Sadness (which the first part of the definition explains), the nature of Desire.