That which so disposes the human body that it can be affected in many ways, or which renders it capable of affecting external bodies in many ways, is profitable to man, and is more profitable in proportion as by its means the body becomes better fitted to be affected in many ways, and to affect other bodies; on the other hand, that thing is injurious which renders the body less fitted to affect or be affected.
Whatever is effective to preserve the proportion of motion and rest which the parts of the human body bear to each other is good, and, on the contrary, that is evil which causes the parts of the human body to have a different proportion of motion and rest to each other.
In what degree these things may injure or profit the mind will be explained below. Here I observe merely that I understand the body to die when its parts are so disposed as to acquire a different proportion of motion and rest to each other. For I dare not deny that the human body, though the circulation of the blood and the other things by means of which it is thought to live be preserved, may, nevertheless, be changed into another nature altogether different from its own. No reason compels me to affirm that the body never dies unless it is changed into a corpse. Experience, indeed, seems to teach the contrary. It happens sometimes that a man undergoes such changes that he cannot very well be said to be the same man, as was the case with a certain Spanish poet of whom I have heard, who was seized with an illness, and although he recovered, remained, nevertheless, so oblivious of his past life that he did not believe the tales and tragedies he had composed were his own, and he might, indeed, have been taken for a grown-up child if he had also forgotten his native tongue. But if this seems incredible, what shall we say of children? The man of mature years believes the nature of children to be so different from his own, that it would be impossible to persuade him he had ever been a child, if he did not conjecture regarding himself from what he sees of others. But in order to avoid giving to the superstitious matter for new questions, I prefer to go no farther in the discussion of these matters.
II
Value of Joy and Sorrow
Joy is an emotion by which the body's power of action is increased or assisted. Sorrow, on the other hand, is an emotion by which the body's power of action is lessened or restrained, and therefore joy is not directly evil, but good; sorrow, on the other hand, is directly evil.
III
The Good Emotions
Cheerfulness is joy, which, in so far as it is related to the body, consists in this, that all the parts of the body are equally affected, that is to say, the body's power of action is increased or assisted, so that all the parts acquire the same proportion of motion and rest to each other. Cheerfulness, therefore, is always good, and can never be excessive. But melancholy is sorrow, which, in so far as it is related to the body consists in this, that the body's power of action is absolutely lessened or restrained, and melancholy, therefore, is always evil.
Pleasurable excitement is joy, which, in so far as it is related to the body, consists in this, that one or some of the parts of the body are affected more than others. The power of this emotion may, therefore, be so great as to overcome the other actions of the body. It may cling obstinately to the body; it may impede the body in such a manner as to render it less capable of being affected in many ways, and therefore may be evil. Again, pain, which, on the contrary, is sorrow, considered in itself alone cannot be good. But because its power and increase is limited by the power of an external cause compared with our own power, we can therefore conceive infinite degrees of strength of this emotion, and infinite kinds of it, and we can therefore conceive it to be such that it can restrain an excess of pleasurable excitement, and so far (by the first part of this proposition) preventing the body from becoming less capable. So far, therefore, will pain be good.