16 May – 28 June 1643
Descartes’ thesis of mind-body interaction has been the most perplexing aspect of his philosophy. On 16 May 1643, Princess Elizabeth wrote to her philosophical mentor, Descartes, regarding this very problem. The exchange became the basis for Descartes’, The Passions of the Soul, (1649) written at the urging of Princess Elizabeth. She writes:
For it seems that all determination of movement takes place by the propulsion of the thing moved, by the manner in which it is propelled by that which moves it, and by the qualification and shape of the surface of this latter. Contact is required for the first two conditions, and extension for the third. You yourself entirely exclude extension from the notion you have of mind, and a touching seems to me incompatible with an immaterial thing.
In his letter of 31 May 1643, Descartes replies:
I can truthfully say that this question which your Highness proposes seems to me to be the question which above all others can most reasonably be raised, in sequel to [what I have said in] my published writings. For there are two things in the human soul upon which all the knowledge we can have or on nature depends, on the one hand that it thinks, and on the other that being united to the body it can act and suffer along with the body. I have said [in the Meditations on First Philosophy] almost nothing of this latter, and have studiously set myself to expound only the former. The reason for my doing so is that inasmuch as my principal design was to prove the distinction subsisting between mind and body, the former could serve in this design, whereas the other, if dwelt on, would have been by no means helpful. But as your Highness is so clear-seeing that there is no concealing anything from her, I shall here endeavor to explain the manner in which I conceive the union of mind and body, and how the mind has the power of moving the body.
First, then, I consider that there are in us certain primary notions, which are, as it were, the originals on the pattern of which we form all the rest of our knowledge. And there are only a very few such notions; for after the most general, those of being, of number, of duration, etc., which apply to everything that we can cognize, we have, for body in particular, no notion save that of extension, from which follow those of shape and movement; and for the soul by itself, we have no notion save that of thought, in which are comprised the cognitions of the understanding and the inclinations of the will; finally, for soul and body [operating] together we have no notion save that of their union, and it is on this notion of their union that we have to depend for our notion of the force which the soul has of moving the body, and which the body has of acting on the soul, thereby causing its sentiments and passions.
I consider also that all human science consists simply in distinguishing these notions, and in attributing each of them only to those things to which they pertain. For when we seek to explain any difficulty by means of a notion which does not apply to it, we cannot fail to deceive ourselves, as also when we seek to explain one of these notions by another; being primary, each of them can be understood only by itself. And since our habitual use of the senses has rendered the notions of extension, of shapes and movements, so much more familiar to us than our other notions, the chief cause of our errors is that we ordinarily seek to make use of these notions in explaining things to which they do not apply, as when, in seeking to apprehend the nature of the soul, we look for help to the imagination, or when, in our endeavor to envisage the action of the soul on the body, we view it in the manner of the action of a body on another body.
This is why, in the Meditations, which your Highness has condescended to honor me by reading, my [chief] endeavor has been to treat of the notions which pertain to the soul alone. Consequently, in sequel thereto, the questions with which I must now deal is our manner of apprehending those notions which pertain to the union of soul and body, as distinguished from those which pertain to body alone or to soul alone. For this purpose, we can, I think, make use of what I have written at the close of my Replies to the Sixth Objections. We may not seek for these simple notions otherwise than in our soul, which has them all in itself by its very nature, but which does not always distinguish them sufficiently from one another, or, it may be, fails to attribute them to the subjects to which they ought to be attributed.
Thus I believe that we have hitherto confounded the notion of the force with which the soul acts on the body with that by which one body acts on another, and that we have attributed both of these notions, not to the soul, since we have not yet come to know it, but to the diverse qualities of bodies, such as weight, heat, etc., which we have imagined to be real, that is to say to have an existence distinct from that of body, and consequently to be substances [and, in the case of gravity, to be in effect a self], though we have called them qualities. And in conceiving these qualities we have made use, sometimes of notions which are in us for the knowing of the soul, according as what we have attributed to them has been material or immaterial. For example, on supposing that weight is a real quality of which we have no other notion save that of its being a force to move the body in which it is toward the center of the earth, we have no difficulty in apprehending how it moves this body nor how it is joined to it; nor do we think that it operates by an actual touching of one surface against another, for we experience in ourselves that we have a particular [i.e., a special, additional] notion for use in apprehending it; and I believe that we are misusing this [additional] notion [i.e., of moving force] in applying it to weight, which is nothing really distinct from body, as I hope to show in my physics. This notion [of moving force] has been given us that we may have an awareness of the fashion in which the soul moves the body. . . .
Princess Elizabeth responded to Descartes in another letter in which she makes it clear that he has utterly failed to remove the doubts about the interaction of the two substances. Moreover, the attempt to explain this interaction by way of the notion of gravity has only served to bewilder her. She claims that she finds it easier to allow matter and extension to the mind than to think that an immaterial substance has the capacity to move a body and be moved by it.
Descartes responds in his letter of 28 June 1643:
I am very greatly obliged to your Highness, that after having found that I had explained myself badly in my preceding remarks regarding the question you have been pleased to propound to me, you yet design to have the patience to listen to me further on the same topic, and to give me the opportunity of dwelling on the things I have omitted. Of the omissions the principal seem to me to be these: that after having distinguished three kinds of primary ideas or notions which are known each in its own particular manner, and not by comparison one with another, i.e., the notion we have of the soul, the notion we have of the body, and the notion we have of the union which is between soul and body, I ought to explain the differences there is in these three kinds of notions, and in the operations of the soul by which we have them, and to state the means we have of rendering each of them familiar and easy; and then, in sequence, to explain why I made use of the comparison with weight, and to show that, while we may choose to view the soul as material (for that is what we do in apprehending its union with the body), we none the less still continue to know that it is separable from the body. These, I take it, are the tasks your Highness would have me discharge.
First, then, I note how very different are the three kinds of notions: the soul apprehends itself solely by means of the pure understanding; the body, that is to say extension, shapes and movements, can be known by the understanding acting alone, but much better by the understanding aided by the imagination; and finally the things which pertain to the union of soul and body, can be known only obscurely by the understanding acting alone, or even by the understanding aided by the imagination, but are known very clearly by the senses. Hence it comes about that those who never philosophize, and who make use only of their senses, entertain no doubts that the soul moves the body and that the body acts on the soul. They consider the two as one single thing, that is to say, they apprehend their union; for to apprehend the union of two things is to apprehend them as one single thing. While metaphysical thoughts which bring into exercise the pure understanding serve to render familiar the notion of mind; and while the study of mathematics, which exercises the imagination chiefly in the consideration of shapes and movements, accustoms us to form very distinct notions of body; it is by relying exclusively on the activities and concerns of ordinary life, and by abstaining from metaphysical meditation and concentrating instead on things which exercise the imagination [in mathematics and physics], that we can learn to apprehend the union of soul and body. . . .
The human mind, as it seems to me, is not capable to conceiving distinctly at one and the same moment both the distinctions between soul and body and their union. To do so, we should have to conceive them as one single thing, and at the same time to conceive them as two; and this cannot be done. . . .
But since your Highness declares that it is much easier to attribute matter and extension to the soul than to attribute to it the capacity to move a body and to be moved by it [i.e., to be sensuously affected] without being itself material, I beg her to feel quite free to attribute to the soul this matter and this extension; for that is precisely what we do in apprehending it as united to the body. And after having viewed them in this way, and having experienced the union in herself, it will be easy for her to recognize that the matter she will have thus attributed to this thought is not the same as thought [i.e., not the same as the soul] and that the extension of this matter is of a different nature from any extension that can be attributed to thought. For whereas the extension of matter is determined to a certain location from which it excludes all other corporeal extension, this does not hold of the extension appropriate to thought [i.e., to mind].
(Translated by Norman Kemp Smith)