Descartes & the Passions of the Soul
René Descartes wrote a book, The Passions of the Soul, 1 presents a strong view of dualism. In this sense, it is similar to Plato's view of the soul, where Plato posited a soul who guides the body much like a helmsmen who pilots a ship.
Descartes began by correctly noting there is an important difference between animate and inanimate bodies. The difference between them must be attributed to the soul. This seems like a valid way to differentiate between the two classes of bodies.
3. The rule we must follow in order to do this
We shall not find this very difficult if we bear in mind that anything we experience as being in us, and which we see can also exist in wholly inanimate bodies, must be attributed only to our body. On the other hand, anything in us which we cannot conceive in any way as capable of belonging to a body must be attributed to our soul. 2
However, then he violates his own standard and limits the soul to just our thoughts. While it is clear that thoughts are one way to differentiate between an animate and inanimate body, yet there are other differences between these bodies too, such as, vegetative and sensitive acts. Some animated bodies may not have thoughts, but they are still animated, such as living plants.
7. A brief account of the parts of the body and of some of their functions
....Finally, it is known that all these movements of the muscles, and likewise all sensations, depend on the nerves, which are like little threads or tubes coming from the brain and containing, like the brain itself, a certain very fine air or wind which is called the 'animal spirits'. 3
10. How the animal spirits are produced in the brain
....For what I am calling 'spirits' here are merely bodies: they have no property other than that of being extremely small bodies which move very quickly4...
17.The functions of the soul
Having thus considered all the functions belonging solely to the body, it is easy to recognize that there is nothing in us which we must attribute to our soul except our thoughts.5
Descartes argued that the immaterial human soul acts upon the physical body via the pineal gland. He correctly noted that the soul is a unity. Even though we have two eyes and two ears, we have a single mental visual impression. This observation led him to postulate there had to be a single physical organ that was an intermediary between the soul and the body. Since he believed that the brain was the center of mental activity, so he posited the pineal gland as that single physical organ, because it is a single organ near the center of the brain.
31. There is a little gland 1 in the brain where the soul exercises its functions more particularly than in the other parts of the body
We need to recognize also that although the soul is joined to the whole body, nevertheless there is a certain part of the body where it exercises its functions more particularly than in all the others. It is commonly held that this part is the brain, or perhaps the heart - the brain because the sense organs are related to it, and the heart because we feel the passions as if they were in it. But on carefully examining the matter I think I have clearly established that the part of the body in which the soul directly exercises its functions is not the heart at all, or the whole of the brain. It is rather the innermost part of the brain, which is a certain very small gland situated in the middle of the brain's substance and suspended above the passage through which the spirits in the brain's anterior cavities communicate with those in its posterior cavities. The slightest movements on the part of this gland may alter very greatly the course of these spirits, and conversely any change, however slight, taking place in the course of the spirits may do much to change the movements of the gland.
1The pineal gland, which Descartes had identified as the seat of the imagination and the 'common' sense in the Treatise on Man (CMS I 106).
32. How we know that this gland is the principal seat of the soul
Apart from this gland, there cannot be any other place in the whole body where the soul directly exercises its function. I am convinced of this, by the observation that all the other parts of our brain double, as also are all the organs of our external senses — eyes, hands, ears and so on. But in so far as we have only one simple thought about a given object at any one time, there must necessarily be some place where the two images coming through the two eyes, or the two impressions coming from a single object through the double organs of any other sense, can come together in a single image or impression before reaching the soul, so that they do not present to it two objects instead of one. We can easily understand that these images or other impressions are unified in this gland by means of the spirits which fill the cavities of the brain. But they cannot exist united in this way in any other place in the body except as a result of their being united in this gland.6
Descartes did not take into consideration Aristotle's critique of Plato's view of the soul. So, in many ways, Descartes' view of the soul falls under Aristotle's criticism too. The soul is the principle of life which is more than a principle of thought.
1 Descartes, Rene (1596-1650), The Passions of the Soul, In: Selected Philosophical Writings, Translated by: J. Cottingham, R. Stoothoff, and D. Murdoch, Cambridge University Press, Cambridge, England, 1988, pp. 218-238.
2 ibid. 
3 ibid. 
4 ibid. 
5 ibid. 
6 ibid. [352-353]