“I beseech you to tell how the soul of man (since it is but a thinking substance) can determine the spirits of the body to produce voluntary action. For it seems every determination of movement happens from an impulsion of the thing moved, according to the manner in which it is pushed by that which moves it, or else, depending on the qualification and figure of the superficies of this latter. Contact is required for the first two conditions, and extension for the third. You entirely exclude extension from your notion of the soul, and contact seems to be incompatible with an immaterial thing.”

“I can in all honesty say that the question Your Highness proposes seems to me that which can be asked with the greatest justification in sequel to the writings I have published. For, there being two things in the human soul (compare: animal soul) on which depends all the knowledge we can have of its nature—the first, that it thinks, and the second, that being united to the body, it can act and suffer with it—I have said nearly nothing of this latter, and have studied only to understand well the first, since my principal design was to prove the distinction that exists between the soul and the body, for which the first alone could suffice, while the other would have been an impediment. But since Your Highness is so discerning that one cannot hide anything from her, I shall try here to explain the manner in which I conceive the union of the soul with the body, and how it has the force to move the body.

Firstly, I consider that in us are certain primitive notions that are like originals on whose model we form all our other knowledge. And there are but very few such notions; for, after the most general ones—of being, number, duration, etc.—which refer to everything we can conceive, we have, as regards body in particular, only the notion of extension, from which follow those of figure and movement; and, as regards the soul alone, we have only that of thought, in which are comprised the perceptions of the understanding and the inclinations of the will; finally, for the soul and the body together, we have only that of their union, on which depends that of the force of the soul for moving the body, and of the body for acting upon the soul by causing its feelings and passions.

I consider also that all human knowledge consists only in carefully distinguishing these notions, and in attributing each of them only to the things to which they pertain. For when we wish to explain some difficulty by means of a notion that does not pertain to it, we cannot fail to make a mistake. And that occurs whenever we wish to explain one of these notions by another—for since they are primitive, each of them cannot be understood except through itself. And inasmuch as the use of the senses has rendered the notions of extension, figures, and movements very much more familiar to us than the others, the principal cause of our errors consists in that we ordinarily wish to employ them to explain things to which they do not pertain, as when one wishes to employ the imagination to conceive the nature of the soul, or else, when one wishes to conceive the manner in which the soul moves the body after the fashion in which a body is moved by another body.

That is why, having tried to clarify in the Meditations Your Highness has deigned to read, the notions that pertain to the soul alone and distinguish them from those that pertain to the body alone, the first thing I should in sequel explain is the manner of conceiving whatever pertains to the union of the soul with the body, leaving aside things that pertain to the body alone or to the soul alone. In this regard it seems to me that what I wrote at the end of my Response to the sixth objections can be of use; for we cannot seek these simple notions other than in our soul—in our soul which, although it has all of them in it by its nature, does not always sufficiently distinguish them one from another, or else fails to attribute them to the objects to which they should be attributed.

Thus I believe that we hitherto confused the notion of the force by which the soul acts on the body with that by which one body acts upon another; and that we have attributed both, not to the soul, for as yet we did not recognize it, but to different qualities of bodies, such as weight, heat, and so forth which we imagined as being real—or, as having an existence distinct from that of body, and consequently as being substances, although we called them qualities. And in order to conceive them, we have sometimes used the notions that are in us for knowing body, and sometimes those that are for knowing the soul, according as what we attributed to them has been material or immaterial. For example, in supposing weight a real quality, of which we possess no other knowledge save that it has the force of moving the body in which it exists toward the center of the earth, we have no difficulty conceiving how it moves this body, nor how it is joined to it; and we do not think that happens by means of an actual touching of one surface against another, for we experience in our own selves that we have a particular notion for conceiving it; yet I believe that in applying this notion to weight—which, as I hope to show in physics, is nothing really distinct from body—we are abusing what has been given us for conceiving the manner in which the soul moves the body.

I would show myself insufficiently aware of the incomparable wit of Your Highness if I employed more words in ex-plaining myself, and I would be too presumptuous if I dared think my response ought satisfy her entirely; but I shall try to avoid both by adding nothing further here save that, if I am capable of writing or saying anything that can be agreeable to her, I will always consider it a very great favor to take up my pen or to go to the Hague on such account; and that there is nothing in the world so dear to me as to obey her commands. But I can find no room here for observing the oath of Hippocrates as she enjoins me; for she has communicated nothing to me that does not merit being seen and admired by all. I can only say, regarding this matter, that infinitely esteeming your letter, I shall treat it as misers do their treasures—which they stash away all the more they esteem them, and, by begrudging everyone else the sight of them, take their sovereign contentment in looking upon them. And thus I shall be very willing to enjoy all to myself the good of looking upon it; and my greatest ambition is to be able to say, and truly to be, . . . etc.”