[223] There is nothing one can more profitably occupy oneself with than with trying to know oneself. And the usefulness one should expect from this knowledge concerns not only morality, which is what many people think of first, but also in particular it concerns medicine, in which I believe one could find many reliable precepts, both to cure and to prevent diseases, and even [224] to slow down the onset of senility, if one had sufficiently made it one’s study to know the nature of our body, and had never attributed to the soul the functions which depend only on the body and the disposition of its organs.

But since we have all from childhood learned by experience that many of these movements obey the will, which is one of the powers of the soul, this has disposed us to believe that the soul is the origin of them all. In addition, ignorance of anatomy and mechanics has contributed much to this: for, in considering nothing but the exterior of the human body, we have never imagined that it contained enough organs or springs to move of its own accord in as many different ways as we actually see it move. And this error has been confirmed by our judgment that corpses are completely devoid of movement, even though they have the same organs as living bodies, and that the only thing they lack is the soul.

However, provided we make the effort to know our nature more distinctly, we can see that our soul, since it is a substance distinct from the body, is known to us only by the single fact that it thinks, that is to say, that it understands, that it wishes, that it imagines, that it remembers, and that it feels, since all these functions are types of thought. Moreover, since the other functions that some people attribute to it, like moving the heart and the arteries, digesting food [225] in the stomach, and other such activities involving no thought, are purely bodily motions; and since it is more natural that a body should be moved by another body than by a soul, we can see that we have less reason to attribute them to the soul than to the body.

We can also see that when certain parts of our body are injured (when a tendon is cut, for example), this brings it about that they no longer obey our will as they had been accustomed to, and even that they often undergo convulsive movements that are contrary to our will. This shows that the will cannot excite any movement in the body unless all the bodily organs necessary for such a movement are appropriately disposed; indeed that, on the contrary, as long as the body has all its organs disposed to a certain movement, there is no need of the soul to produce it; and that consequently all the movements which we do not experience as depending on our thought should not be attributed to the soul, but to the disposition of the organs alone; and that even the movements which are called ‘voluntary’ proceed principally from this disposition of the organs, since they cannot be excited without it, whatever we may will about them, even if it is the soul that determines them.

And although all these movements in the body cease when it dies and the soul leaves it, one must not infer from this that it is the soul that produces them, but only that it is one and the same cause which brings it about that the body is no longer fitted to produce them, and which also brings it about that the soul absents itself from the body.

[226] It is true that one can have difficulty in believing that the disposition of the organs alone is sufficient for producing in us all the movements which are not determined by our thought; that is why I shall here try to prove it; and to explain the whole machine of our body in such a way that we shall have no more reason to think that it is our soul which excites in it the movements which we do not experience as being guided by our will, than we have to judge that there is a soul in a clock, which makes it show the hours.

There is no one who does not already have some knowledge of the various parts of the human body, that is to say, who does not know that it is composed of a very large number of bones, muscles, nerves, veins, arteries, together with a heart, a brain, a liver, lungs, a stomach; and even who has not sometimes seen various animals opened up, on which occasions they have been able to observe the shapes and positions of their internal parts, which are approximately the same in them as in us. It will be unnecessary to have any more anatomical knowledge than this in order to understand this book, because I shall take care to explain everything more detailed that needs to be known, as soon as I have occasion to talk of it.

And so that the reader will have from the beginning a general notion of the whole machine which I have to describe, I shall say here that it is the heat of the heart which is so to speak the mainspring and origin of all the movements of the body; and that the veins are the pipes which carry the blood from all parts of the body towards the heart, where it serves as nourishment for the heat which is there, [227] as also the stomach and the guts are another bigger pipe, dotted with many little holes through which the juice of the food runs into the veins, which carry it straight to the heart. And the arteries are yet more pipes, through which the blood, warmed and rarefied in the heart, passes from there into all the other parts of the body, to which it carries the warmth and the matter needed for nourishment. And finally the most agitated and lively parts of the blood, having been carried to the brain by the arteries which come from the heart by the straightest route of all, compose a sort of air, or very subtle wind, which is called the Animal Spirits; which, dilating the brain, make it capable of receiving the impressions of external objects and of the soul, that is to say, of being the organ or seat of the Common Sense, the Imagination, and the Memory. Then this same air or these same spirits run from the brain through the nerves into all the muscles, whereby they make these nerves fit to serve as instruments of the external senses, and, inflating the muscles in various ways, give movement to all the members of the body.

That is a summary of all the things I have to describe here for us to be able to know distinctly what there is in each of our actions that depends only on the body, and what there is that depends on the soul. With this knowledge, we will be in a better position to make good use of both of them, and to cure or prevent the illnesses they are liable to.