P11: The idea of any thing that increases or diminishes, aids or restrains, our Body’s power of acting, increases or diminishes, aids or restrains, our Mind’s power of thinking.

Dem.: This proposition is evident from IIP7, or also from IIP14.

Schol.: We see, then, that the Mind can undergo great changes, and pass now to a greater, now to a lesser perfection. These passions, indeed, explain to us the affects of Joy and Sadness. By Joy, therefore, I shall understand in what follows that passion by which the Mind passes to a greater perfection. And by Sadness, that passion by which it passes to a lesser perfection. The affect of Joy which is related to the Mind and Body at once I call Pleasure or Cheerfulness, and that of Sadness, Pain or Melancholy.

But it should be noted {NS: here} that Pleasure and Pain are ascribed to a man when one part of him is affected more than the rest, whereas Cheerfulness and Melancholy are ascribed to him when all are equally affected.

Next, I have explained in P9 S what Desire is, and apart from these three I do not acknowledge any other primary affect. For I shall show in what follows that the rest arise from these three. But before I proceed further, I should like to explain P10 more fully here, so that it may be more clearly understood how one idea is contrary to another.

In IIP17 S we have shown that the idea which constitutes the essence of the Mind involves the existence of the Body so long as the Body itself exists. Next from what we have shown in IIP8 C and its scholium, it follows that the present existence of our Mind depends only on this, that the Mind involves the actual existence of the Body. Finally, we have shown that the power of the Mind by which it imagines things and recollects them also depends on this (see IIP17, P18, P18 S), that it involves the actual existence of the Body.

From these things it follows that the present existence of the Mind and its power of imagining are taken away as soon as the Mind ceases to affirm the present existence of the Body. But the cause of the Mind’s ceasing to affirm this existence of the Body cannot be the Mind itself (by P4), nor also that the Body ceases to exist. For (by IIP6) the cause of the Mind’s affirming the Body’s existence is not that the Body has begun to exist. So by the same reasoning, it does not cease to affirm the Body’s existence because the Body ceases to exist, but (by IIP8) this {sc. ceasing to affirm the Body’s existence} arises from another idea which excludes the present existence of our body, and consequently of our Mind, and which is thus contrary to the idea that constitutes our Mind’s essence.