P39: Those things are good which bring about the preservation of the proportion of motion and rest the human Body’s parts have to one another; on the other hand, those things are evil which bring it about that the parts of the human Body have a different proportion of motion and rest to one another.

Dem.: To be preserved, the human Body requires a great many other bodies (by IIPost.4). But what constitutes the form of the human Body consists in this, that its Parts communicate their motions to one another in a certain fixed proportion (by the Definition).

Therefore, things which bring it about that the Parts of the human Body preserve the same proportion of motion and rest to one another, preserve the human Body’s form. Hence, they bring it about that the human Body can be affected in many ways, and that it can affect external bodies in many ways (by IIPost. and Post. 6). So they are good (by P38).

Next, things which bring it about that the human Body’s parts acquire a different proportion of motion and rest to one another bring it about (by the same Definition) that the human Body takes on another form, i.e. (as is known through itself, and as I pointed out at the end of the preface of this Part), that the human Body is destroyed, and hence rendered completely incapable of being affected in many ways. So (by P38), they are evil, q.e.d.

Schol.: In Part VI shall explain how much these things can be harmful to or beneficial to the Mind. But here it should be noted that I understand the Body to die when its parts are so disposed that they acquire a different proportion of motion and rest to one another. For I dare not deny that—even though the circulation of the blood is maintained, as well as the other {signs} on account of which the Body is thought to be alive—the human Body can nevertheless be changed into another nature entirely different from its own. For no reason compels me to maintain that the Body does not die unless it is changed into a corpse.

And, indeed, experience seems to urge a different conclusion. Sometimes a man undergoes such changes that I should hardly have said he was the same man. I have heard stories, for example, of a Spanish Poet who suffered an illness; though he recovered, he was left so oblivious to his past life that he did not believe the tales and tragedies he had written were his own. He could surely have been taken for a grown-up infant if he had also forgotten his native language.

If this seems incredible, what shall we say of infants? A man of advanced years believes their nature to be so different from his own that he could not be persuaded that he was ever an infant, if he did not make this conjecture concerning himself from {NS: the example of} others. But rather than provide the superstitious with material for raising new questions, I prefer to leave this discussion unfinished.