In other words, like Plato, the body is inferior and its substance is irrelevant for true and certain knowledge. The intellect with its faculties (judgment, imagination, memory, free will, etc.) is most important.
The sixth meditation is the crucial one. He shows the body as "an extended, non-thinking thing" (VII: 78). This is accepted as being close to who he is, but not as close as the mind part. "And accordingly," he says, "it is certain that I am really distinct from my body, and can exist without it" (VII: 78). In other words, the mind and the body are separate, not dependent on each other. This is not exactly an argument for the immortality of the soul in the Platonic way. but, as Wilson says, "He now determines that there is no reason why the death or destruction of the body should entail the death or destruction of the mind" (Wilson 177). It is logically possible. Furthermore, the mind is divided into "modes of thinking" such as imagination and sensory perception. These are different forms of mental substance. They are different than corporeal or extended substance. Descartes has a more realistic view of the soul as mind, and does not disparage the body like Plato does. However, he continues the priority of mind over matter, and a dualism that puts them as totally different substances whose connection is mysterious.
So how are mental and corporeal substance connected? His principal example is pain. He feels intellectual distress during physical pain. If there were no essential connection, he would not feel such pain. This proves for him the intermingling of mind and body. He uses additional examples of hunger and thirst. Finally, he says he is affected unfavorably by bodies close to his sometimes. This tells him that "my body, or rather my whole self, in so far as I am a combination of body and mind, can be affected by the various beneficial or harmful bodies which surround it" (VII:81). This is a kind of partnership. At the same time, it seems to contradict the clear substantial distinction of body and mind. So Wilson says, "The intermingling is an as-it-were intermingling, an impression as of intermingling that is itself an idea" (Wilson 203). That is, it is a real union, but one only in thought.
Using examples of fire and stars, perceived heat and light, he shows that "the proper purpose of the sensory perceptions given me by nature is simply to inform the mind of what is beneficial or harmful for the composite of which the mind is a part" (VII: 83). They need not resemble the actual state outside of him, as though the mind held copies of them. Heat and pain are not in the external things but in the perceiving body. This shows how he views the relationship of mind and body, and the priority he places on the mind. The mind is the place for knowledge, not the physical body. Meanwhile, he seems to take a rather mechanical view of the human body, comparing it with a clock that operates by the laws of nature. He says he may consider the body as a machine that could perform all the same movements as it would when controlled by a will or mind. The body receives and reacts to the environment even without the mind. Here he also seems to link sense experience with action, or dispositions to act. Things show us what is useful or not to do or to respond. They are signals. Obviously, the will or mind is what animates the body, but not what determines whether or not a body gets thirsty or sick. So there is some concept of "nature" (or God) that Descartes uses to explain the body apart from the mind.
Further, the body is divisible by nature, whereas the mind is absolutely indivisible. One can cut off an arm and a part of the mind does not disappear. And the faculties are united, unlike in Plato where they are composite and in conflict: "it is one and the same mind that wills, and understands and has sensory perceptions" (VII: 86). He goes on to localize the mind in the brain, unlike Plato who disperses its separate parts. It is affected only by the part of the brain that contains common sense. Yet the brain is linked to all parts of the body through nerves. To this extent, Descartes demonstrates greater familiarity with physiology than Plato, and how sensations are distributed through the body. He conceives of these as signals (we would say impulses) that produce one corresponding sensation and are conducive to health.
Descartes's view suffers from the same problem as Plato's. While not immortality, the notion that the mind can exist without a body is not proven. It is conceivable, but in imagination only. In reality, nothing shows the existence of minds without bodies. It seems more likely that minds need bodies. His appeal to God does not work today. He cannot show how thought is possible without a body or brain. Descartes says it is thinking, not imagination or sensory perception that could exist without a body. This still does not solve the problem. Even if something like immaterial substance exists, would it not just as likely disintegrate into parts after death just like the body? All things decay. This includes mental as much as physical. Any notion of immortality or imperishable mind substance abstract from a physical body is suspect. From what we understand now, thought is a product of physical processes. Without a body, there is no thought. Mind cannot function outside a connection with material, so neither the Platonic nor the Cartesian positions make sense. It is the body that thinks. Thought is a function of biology, not of imaginary immaterial being. That we can even conceive of a soul or mind outside of the body is no more than a product of embodied thinking.
Neither Plato nor Descartes solves the relationship between body and mind. While this interrelation is difficult to understand, perhaps even irresolvable, something along materialistic lines seems most plausible. This allows us not to consider the bodily functions and desires as below the mental, thus escaping moral judgmentalism or any notion of sin and natural depravity. It also lets us link soul/mind and body by seeing them as part of the same substance. There need not be a dualism. The inhabitation of the inferior body with a superior mind becomes an unnecessary idea. Rather, it is precisely the body and its intricate make-up that gives rise to thought, imagination, desire, memory, and emotions. The body creates the soul. The two are not separate. In other words, an Aristotelian view may be the more correct answer R.G. Swinburne writes, "For Aristotle, the soul was simply the form of the body, i.e. The way the body behaved, and thus not capable of existing separately from it; plants and animals also had souls of their own kinds" (Swinburne 841). While the relationship is still mysterious either way, the evidence for the soul seems to suggest that its existence relies on the presence of a body that animates it.
Annas, Julia. Plato: A Very Short Introduction. Oxford: Oxford University Press, 2003.
Descartes, Rene. Meditations on First Philosophy with Selections from the Objections and Replies. Trans. And ed. John Cottingham. Cambridge: Cambridge University Press, 1996.
Kim, Jaegwon. "Mind-body problem, the." In the Oxford Companion to Philosophy, ed. Ted Honderich, 579-580. Oxford: Oxford University Press, 1995.