The Meditations represent Descartes effort to defeat skepticism by showing that if one proceeds to systematically consider the matter, there are truths which one simply cannot doubt. He does this by using a method of doubt and a method of analysis jointly. The method of doubt involves posing more and more powerful skeptical hypotheses which call into doubt classes of knowledge claims. (The Cartesian method of doubt gives some more detail in regard to this method.) These classes are determined by the three faculties by which Descartes holds we might come to know things, the senses, the imagination and reason or understanding. Having eleminated all truths derived from the senses and the imagination and many of those which come from reason, Descartes finally concludes that he cannot be deceived into thinking that he does not exist when in fact he does, for unless something existed there would be nothing to deceive. So, he has found one truth which he can know with absolute certainty.
Descartes method here has impressed many philosophers. The project of finding a certain foundation on which to base all knowledge claims is one which has endured well into the twentieth century. Some twentieth century philosophers have challenged both Descartes' method and the whole project of foundationalism. One of the problem raised by these philosophers is the problem of the incoherent skeptical hypotheses.
In Meditation II Descartes finds other truths which he knows for certain. He discovers that not only does he exist but that he is essentially a thinking thing (sum res cogitans). (You might consider how this is different from simply knowing that you exist.) Descartes also discovers that the essence of material bodies (if any exist -- something which he doesn't know at this point!) is to be flexible, movable and extended. The arguments here (in the famous wax example) are similar to and yet different from skeptical arguments about the nature of bodies. They are similar in that Descartes agrees with the skeptic about what does and does not constitute knowledge and how you tell. The difference is that Descartes thinks that there are some properties of things which we can know that they have essentially, and thus know that they have them for certain.
At the beginning of Meditation III Descartes finds a whole host of truths which he holds we can know for certain. These truths involve the causal or representational theory of perception. This theory holds that we directly perceive ideas which are caused by objects in the external world. Descartes claims that we can know for certain that we are seeing a particular idea (of the sun or the stars or this room or that tree), what we don't know for certain is if there is a sun or stars or a room or tre e causing our ideas). Recently commentators have argued that Descartes does not hold the causal theory of perception. This also creates an interesting philosophical problem. Who has got it right -- the commentators who insist that Descartes is an adherent of the causal theory of perception or the new commentators who deny that he does hold this view -- or at least the old "veil of perception" interpretation of it.
Descartes goes on to produce a criterion for truths which we can know for absolute certainty. He does this by reflecting on those truths which he has already discovered using the method of doubt, and determines that what they all have in common is that the ideas in them are all clear and distinct. Thus any truth composed of clear and distinct ideas can be known for certain. Descartes then proceeds to try to move from the foundation, to determine what truths might be based on those truths. The first thing he must do, as it turns out is to prove that God exists! Without doing this he cannot get rid of the Evil Demon hypothesis. By proving that God exists he recovers the truths of mathematics!
The proof of the existence of God is complicated and involves scholastic technical terminology with which modern readers tend to be quite unfamiliar. I pursue this topic in Descartes' Proof for the Existence of God It is worth noting, however, how clever Descartes is, given the serious constraints which are imposed upon him by his own way of proceeding, in finding the materials to construct a proof of God's existence at this point. However one ultimately judges the soundness of the proof, the effort itself shows real philosophical genius!
I shall not here pursue in detail Descartes' ascent from the Foundation up through the truths of mathematics, to the application of mathematics to objects in the world, to proofs of the existence of external objects. Rather I shall turn to Descartes' account of the relation of the mind and the body. This involves three topics --- mind body dualism, proofs for the real distinction between mind and body, and two way causal interaction.
Descartes in Meditation II concludes that he is in essence a thinking thing (res cogitans), and that it is possible that he exists without a body. He recognizes, however, that to conclude from this that his mind is really distinct from his body would be fallacious. The Stoic paradox of the masked man illustrates the fallacy. If a person sees their father, they will very likely recognize him. If the same person is then shown a masked man, they may doubt that this is their father. Still, the masked man might be their father. So, to conclude that the father and the masked man are really distinct because one cannot doubt in one case but can in the other is clearly fallacious. The case is precisely the same for Descartes in relation to his mind and his body in Meditation II.
Descartes does make some further progress in respect to this problem in Meditation II -- he determines, as noted above, that the essence of body is to be flexible, movable and extended. Thus it turns out that the essence of mind and the essence of body are two different things. Minds are in essence thinking things, bodies are in essence space occupiers, movable and flexible. This is the basic doctrine of Cartesian dualism with respect to mind and body -- bodies and minds are different kinds of entities. One motivation for this distinction is an interest in the Christian doctrine of immortality. Descartes originally intended the Meditationes de Prima Philosophia to include a proof of the immortality of the soul. He does not entirely succeed in this, but perhaps takes some steps towards the achievement of such a goal. One of these steps involves proving that there is a real distinction between the mind and the body, i.e. that the soul can exist independently of the body.
In Meditation VI Descartes gives two proofs for the real distinction between minds and bodies. One of these involves a contrast between the simple nature of the soul and the complex nature of the body. The other picks up the materials which Descartes had provided for himself in Meditation II and III and puts them together into a proof. From Meditation II Descartes takes the fact that the essences of mind and body are distinct. From Meditation III he takes the language of clear and distinct ideas, and the existence of God and from God's omnipotence concludes that God could make distinct any two things which I clearly and distinctly perceive could be distinguished. Since the essence of mind and body are different and these ideas are clear and distinct, it follows that God could make them distinct. Descartes concludes that they are in fact distinct. There is a debate in the scholarly literature over whether this last step is justified. This also is a philosophical problem about which one might write a paper.
Supposing that minds and bodies really are distinct from one another, how do they relate to one another? Minds are thinking things which are not extended, bodies are extended and do not think. Descartes answer is that minds are affected by bodies in perception and that bodies are affected by minds in action. Thus, when I see a tree, the tree is causing light rays to hit my eye, this information is taken by the animal spirits up to the brain, and passed through the pineal gland to the mind where it is perceived as the idea of a tree. On the other hand, if I decide to lift my arm, my mind issues a command which is passed through the pineal gland to the brain, and from the brain the animal spirits are animated in such a way that my arm raises. This is two way causal interaction. It is two way because the mind causally affects the body in action, and the body causally affects the mind in perception. It is causal because the process is causal and not say logical or aesthetic or some other kind of relationship. It is interaction because it is one kind of entity acting on another, that is minds on bodies or bodies on minds.
After Descartes articulated this theory, philosophers in the next generation (and thereafter) were struck more by the theory's difficulties than what it explained. How can a mind, which is immaterial, cause any kind of change in a body at all? How can a body, which is material and occupies space, affect something which is immaterial and does not occupy space? It is possible to evade some of these difficulties in various ways. However, for Descartes the difficulties are compounded by Descartes heirloom theory of causality which plays a crucial role in Descartes' proof for the existence of God. This theory holds that the effect inherits something from the cause. This explains the connection between cause and effect. Given the real distinction, however, it appears that there is nothing which can play this role for minds and bodies -- what do they have in common? This again is a philosophical problem well worth writing about.