IXB. descartes to princess elizabeth 1
Egmond, 21 May 1643.
... I may truly say that what your Highness is propounding seems to me to be the question people have most right to ask me in view of my published works. For there are two facts about the human soul on which there depends any knowledge we may have as to its nature: first, that it is conscious; secondly, that, being united to a body, it is able to act and suffer along with it. Of the second fact I said almost nothing; my aim was simply to make the first properly understood; for my main object was to prove the distinction of soul and body; and to this end only the first was serviceable, the second might have been prejudicial. But since your Highness sees too clearly for dissimulation to be possible, I will here try to explain how I conceive the union of soul and body and how the soul has the power of moving the body.
My first observation is that there are in us certain primitive notions—the originals, so to say, on the pattern of which we form all other knowledge. These notions are very few in number. First, there are the most general ones, existence, number, duration, etc., which apply to
1 [Corresp., No. 302;  Oeuvres, A.-T., vol. iii, p. 663.—ed.]
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everything we can conceive. As regards body in particular, we have merely the notion of extension and the consequent notions of shape and movement. As regards the soul taken by itself, we have merely the notion of consciousness, which comprises the conceptions (perceptions) of the intellect and the inclinations of the will. Finally, as regards the soul and body together, we have merely the notion of their union; and on this there depend our notions of the soul's power to move the body, and of the body's power to act on the soul and cause sensations and emotions.
I would also observe that all human knowledge consists just in properly distinguishing these notions and attaching each of them only to the objects that it applies to. If we try to explain some problem by means of a notion that does not apply, we cannot help making mistakes; we are just as wrong if we try to explain one of these notions in terms of another, since, being primitive, each such notion has to be understood in itself. The use of our senses has made us much more familiar with notions of extension, shape, and movement than with others; thus the chief cause of our errors is that ordinarily we try to use these notions to explain matters to which they do not apply; e.g. we try to use our imagination in conceiving the nature of the soul, or to conceive the way the soul moves the body in terms of the way that one body is moved by another body.
In the Meditations that your Highness condescended to read, I tried to bring before the mind the notions that apply to the soul taken by itself, and to distinguish them from those that apply to the body taken by itself. Accordingly, the next thing I have to explain is how we are to form the notions that apply to the union of the soul with the body, as opposed to those that apply to the body taken by itself or the mind taken by itself. . . . These simple notions are to be sought only within the soul, which is naturally endowed with all of them, but does not
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always adequately distinguish between them, or again does not always attach them to the right objects.
So I think people have hitherto confused the notions of the soul's power to act within the body and the power one body has to act within another; and they have ascribed both powers not to soul, whose nature was so far unknown, but to various qualities of bodies—gravity, heat, etc. These qualities were imagined to be real, i.e. to have an existence distinct from the existence of bodies; consequently, they were imagined to be substances, although they were called qualities. In order to conceive of them, people have used sometimes notions that we have for the purpose of knowing body, and sometimes those that we have for the purpose of knowing the soul, according as they were ascribing to them a material or an immaterial nature. For example, on the supposition that gravity is a real quality, about which we know no more than its power of moving the body in which it occurs towards the centre of the Earth, we find no difficulty in conceiving how it moves the body or how it is united to it; and we do not think of this as taking place by means of real mutual contact between two surfaces; our inner experience shows (nous expirimentons) that that notion is a specific one. Now I hold that we misuse this notion by applying it to gravity (which, as I hope to show in my Physics, is nothing really distinct from body), but that it has been given to us in order that we may conceive of the way that the soul moves the body.