"[T]he 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; and to this extent they are sufficiently clear and distinct. But I misuse them by treating them as reliable touchstones for immediate judgements about the essential nature of the bodies located outside us." (57-8)
In sum, the "lessons of nature" concern only those things that affect our immediate well-being as corporal beings. These would also include the fact that we each have a body that is affected in both harmful and beneficial ways by other corporal bodies.
According to Descartes, what our natures don't teach us is anything about the essential nature of the bodies that surround us. Here we are only given clues (e.g. this body is perceively different than that one) that must be explained by drawing on our clear and distinct ideas of the properties of bodies. Thus, we are led to understand differences of color or smell in terms of differences in the size, shape, and motion of the parts of the bodies that cause these sensations in us.
As its title suggests, Meditation 6 is concerned with two main topics: "The existence of material things and the real distinction between mind and body."
1. Descartes begins by recounting the result of the last Meditation: I know that material things "are capable of existing, in so far as they are the subject matter of pure mathematics, since I perceive them clearly and distinctly. For there is no doubt that God is capable of creating everything that I am capable of perceiving in this manner; and I have never judged that something could not be made by him except on the grounds that there would be a contradiction in my perceiving it distinctly" (50).
Both our imagination and our senses suggest that our mind is united with a body and that other bodies exist outside of us. Yet neither of these can support a demonstration of the existence of bodies; both are liable to deception/error.
2. What we can be sure of, Descartes claims, is that whether or not we possess a body, there is a real distinction between the mind and the body. This means they must be two distinct substances that are at most contingently united during our life. The argument for this "real distinction" proceeds as follows (54)
(1) I know that everything which I clearly and distinctly understand is capable of being created by God so as to correspond exactly with my understanding of it.
(2) The fact that I can clearly and distinctly understand one thing apart from another is enough to make me certain that the two things are distinct, since they are capable of being separated by God.
(3) Simply by knowing that I exist and recognizing at the same time that absolutely nothing else belongs to my nature or essence except that I am a thinking thing, I can infer that my essence consists solely in the fact that I am a thinking thing.
(4) Thus, "on the one hand, I have a clear and distinct idea of myself, insofar as I am simply a thinking, non-extended thing; and on the other hand, I have a distinct idea of body, insofar as this is simply an extended, non-thinking substance. And accordingly, it is certain that I am really distinct from my body and can exist without it."
3. Descartes now is ready to offer his argument for the existence of bodies (54-5):
(1) I have a passive faculty of sensory perception, i.e. of getting and recognizing ideas of sensible objects.
(2) I could not have such a faculty unless there existed some active power, either in myself or in something else, to produce or make the ideas.
(3) This power certainly cannot exist in me, for it presupposes no action of my intellect; sensory ideas are produced without my cooperation and often against my will.
(4) So, the power inheres in some substance other than myself.
(5) This substance must contain at least as much formal reality as exists objectively in my sensory ideas.
(6) So, this substance is either a body (i.e. a material substance) which contains whatever is contained objectively in my ideas; or it is God or some other creature superior to a body, which contains the reality of my ideas in a higher form (eminently).
(7) But God has given me no faculty to discover the origin of my sensory ideas and a strong inclination to believe that these ideas proceed from bodies.
(8) If God were to cause these ideas in me, either directly or by means of some creature other than bodies, God would be a deceiver.
(9) But God is no deceiver.
(10) I may therefore conclude that there exist bodies which are the causes of my sensory ideas of bodies.
4. There are a couple of important points to note about this argument:
Descartes acknowledges that it lacks the deductive certainty of his proofs of God's existence. In the Synopsis to the Meditations, he writes:
"The great benefit of these arguments [in the Sixth Meditation] is not, in my view, that they prove what they establish -- namely that there really is a world, and that human beings have bodies and so on -- since no sane person has ever seriously doubted these things. The point is that in considering these arguments we come to realize that they are not as solid or as transparent as the arguments which lead us to knowledge of our own minds and of God, so the latter are the most certain and evident of all possible objects of knowledge for the human intellect. Indeed, this is the one thing that I set myself to prove in these Meditations." (11)
Descartes recognizes that have an overwhelming inclination to believe in the existence of external things; thus, there is no need to demonstrate this in order to convince us of the existence of bodies. Instead, the present argument shows that based on what we know through clear and distinct perceptions, our certainty concerning God's existence is justified in a way that our certainty concerning the existence of external material things is not. In the latter case, we begin from the assumption that we do not have clear and distinct perceptions of the existence of bodies. All we have are (adventitious) sensory ideas, which incline us to believe in the existence of external things without proving that they actually exist. The crucial premise in the argument is (7), which links this inclination to our lack of any capacity for discovering whether or not our belief in the existence of bodies is justified. If God had created us like this, and it turned out there actually weren't any external things (i.e. our life was just one long dream), then we would have cause to accuse God of being a deceiver (which we know he cannot be). In this way we show (indirectly) that bodies must be the causes of our sensory ideas..
Descartes does not claim, however, that the argument proves the existence of the external world exactly as we perceive it. Bodies "may not all exist in a way that exactly corresponds with my sensory grasp of them, for in many cases the grasp of the senses is very obscure and confused. But at least they possess all the properties which I clearly and distinctly understand, that is, all those which, viewed in general terms, are comprised within the subject-matter of pure mathematics" (55). All the argument shows it is that some external things (and not God) are the causes of our sensations. Beyond this, our knowledge of what those things are must be based on our clear and distinct intellectual ideas, which have instructed us that the essence of matter is spatial extension.
5. Based on the same reasoning, Descartes concludes that there is some truth in everything he has been "taught by nature," i.e. everything he has a strong inclination to believe and no reliable means of detecting its falsity. The things that "nature teaches me" include:
i) that I have a body;
ii) that when I feel pain there is something wrong with my body;
iii) that when I am hungry or thirsty the body needs food and drink;
iv) that "I am not merely present in my body as a sailor is present in a ship, but that I am very closely joined and, as it were, intermingled with it, so that I and the body form a unit. If this were not so, I, who am nothing but a thinking thing, would not feel pain when the body was hurt, but would perceive the damage purely by the intellect, just as a sailor perceives by sight if anything in his ship is broken. Similarly, when the body needed food or drink, I should have an explicit understanding of the fact, instead of having confused sensations of hunger and thirst. For these sensations of hunger, thirst, pain and so on are nothing but confused modes of thinking which arise from the union and, as it were, intermingling of the mind with the body" (56).
v) that various other bodies exist in the vicinity of my body, and that some of these are to be sought out and others avoided;
vi) that "from the fact that I perceive by my senses a great variety of colors, sounds, smells and tastes, as well as differences in heat, hardness and the like, I am correct in inferring that the bodies which are the source of these various sensory perceptions possess differences corresponding to them, though perhaps not resembling them." (56)
6. At the same time, however, Descartes insists that "there are... many other things which I appear to have been taught by nature, but which in reality I acquired not from nature but from a a habit of making ill-considered judgments." These include the belief that space is empty (i.e. that a vacuum exists); that the heat in a body is something exactly resembling the idea of heat in me; that colors and other sensory qualities are real properties of bodies; and other judgments made about the world solely on the basis of sensation. In order to distinguish these mistaken judgments from the "lessons of nature," we require a clearer sense of what the latter involve. 7. This conclusion, however, gives rise to one final problem. Sometimes, it seems, bodily signals mislead us, leading us to do things that don't further our survival. For example, the person with dropsy will continue to drink beyond what is healthy for him and will eventually die from the disease. In this case, the lessons of nature seem to mislead us in a fatal way.
Descartes answers this objection by further exploring the character of the mind-body relation. Although the mind is united to the whole body, in that it is sensitive to changes in and capable of influencing many different parts of the body, it is nonetheless directly connected to only one small part of the brain, which Descartes claims is the pineal gland or "conarium." The mind's connection to the rest of the body is effected through nerves which (mechanically) transmit messages between the pineal gland and other parts of the body. The system has been created by God so that under normal conditions just the right messages are transmitted: pain in the foot when the foot is injured, hunger when the stomach is empty, etc. However, given the nature of the connection between the mind and the body, it is possible that false messages will sometimes be transmitted when the nerves are stimulated at intermediate points. This explains the false pains experienced by amputees and conditions like dropsy.
8. By the end of the Sixth Meditation, Descartes has come full circle in terms of his attitude toward the senses. Provided that we use them cautiously, "in matters regarding the well-being of the body, all my senses report the truth much more frequently than not" (61). The crucial point, again, is distinguishing the proper function of the senses: They provide us with reliable, immediate information about the condition of our own body (its well-being) but not with scientific knowledge about the nature of bodies in general. In order to acquire sound scientific knowledge, we must rely on our innate, intellectual idea of matter as extension.
After reaching these conclusions, Descartes is prepared to dismiss the doubts that seemed so pressing in the First Meditation: "This applies especially to the principal reason for doubt, namely my inability to distinguish between being asleep and being awake" (61). Now we are able to fall back on our ordinary ways of distinguishing between waking and dreaming experience, knowing that, provided we have used our faculties of sensation, memory and intellect in a responsible manner, we will not go astray. For God is not a deceiver!