According to Descartes we know our own mind better than we know bodies. Why does he think so? How can this view be criticized?
The conclusion that the mind is better known than the body is a consequence of Meditations 1 and 2 from the Meditations on First Philosophy. Descartes explicitly states the claim in a couple different ways. Notice that it is the subtitle of the Second Meditation: "The nature of the human mind, and how it is better known than the body." From this fact we can gather that a chief aim of the Second Meditation is the demonstration of this claim. The short answer to your question, then, is to study the Second Meditation. Let me provide some guidance.
Descartes claims in the Objections and Replies that, "the nature of the mind is the one we know best of all" (72). So what Descartes' claim is is that the nature of the mind is better known than the nature of the body. Hence, it is no objection to Descartes to claim that the body is more viscerally known, or that the body is more intimately experienced in daily life. For just because I experience my own body in very immediate sensations, does not entail that I know the nature of my body better than I know the nature of my mind.
In the famous wax argument from the Second Meditation, Descartes entertains a counter-argument to the claim that I know the nature of my mind better than the nature of body. Up to this point, Descartes has claimed that he is certain of the existence of the I (cogito sum) and that the nature of the I is to be a thinking thing. And Descartes has been able to discover this before he has been able to discover anything about the nature of the material world. But it would be natural to object at this point that this is absurd. For, I can perceive the material world with my senses, but I can't perceive the mind with my senses. I can easily find out about the material world by looking around, whereas perceiving the mind (if it is possible) would require a subtle philosophical inquiry. So the idea that I have a clearer perception of my own mind than of the material world must be false (so the objection would go).
Descartes gives the wax argument to support the findings of the cogito against this kind of objection. Let us take an ordinary material object, such as a piece of wax. Descartes argues that the wax is not known by the senses or the imagination, but rather by the pure intellect. Hence, the material world is not in fact known by a different faculty than that by which the mind is known. Therefore, the objection rests on a faulty assumption. And since the cogito is first in the order of knowledge (as was shown by the cogito), the mind is better known than the body.
Why should I think that the wax is known by the pure intellect? Senses perceive perceptible qualities such as color, shape, smell, and taste. But all of these qualities the wax can lose, while remaining the same wax. For example, the wax is hard, cold, and smells like honey. Then I put it in front of the fire and it becomes gooey, warm, and loses its smell. Yet I know the same wax remains. These sensible qualities are thus accidental features. I must know the nature of the wax by means of knowing its essential qualities. These are qualities it could not lose while remaining what it is. These qualities cannot be fully grasped by the senses, since the senses always give me particular images of things that can vary while nevertheless the wax remains essentially what it is. The essential properties of the wax are its being extended in space and its ability to change into different shapes. But these more general properties are not sensible qualities. Hence, I do not know the nature of the wax itself by means of the senses.
Nor can I know the wax by means of the imagination. The imagination is the faculty of creating sensory images in the mind. What it is for the wax to be extended and changeable is for there to be an infinite number of possible sensible forms that the wax could take up. But it is impossible for me to create an infinite number of images in the mind. Hence, it cannot be that I know the extension and changeability of the wax by means of my imagination.
Rather, what the wax really is, “I perceive through the mind alone” (21), that is, I reason it out as I consider what is common to the whole variety of images under which the wax can be perceived. Thus any perception of a material object can be converted into a perception of the mind. For example: "I see the wax" --> "I see an extended, changable form and my mind judges it to be wax." This activity is itself the nature of the mind to be a thinking thing. As a result, knowing material objects requires already knowing the nature of the mind. Thus I know the nature of my own mind better than I can possibly know the nature of the material world.
All quotations from the Cambridge University Press edition of the Meditations, trans. and edited by John Cottingham.
One criticism would be that the complexity of the mind has been greatly underestimated. Nineteenth-Century psychological researchers thought that examining the conscious mind was sufficient, and Freud encountered a great deal of difficulty in convincing people that there are parts of our minds we aren't aware of. Early AI researchers vastly underestimated the problems of making minds equivalent to humans (Newell and Simon assigned two grad students to implement computer vision over the summer).
So, the mind has changing and unknown properties like the Second Meditation wax, that Descartes was wholly unaware of.
In the Meditation 2, Descartes is sure about his existence because he is sure about there is a deceiver or a supremely powerful God then he concludes that "I am, I exist". The point I don't understand is that in Med 2," If there are not some God, or whatever name I might call him, who instills these very thoughts in me?" then he continues as "But why would I think that, since I myself could perhaps be the author of these thoughts?Am I not then at least something? But I have already denied that I have any senses and any body. Still I hesitate ; for what I follows from this? Am I so tied to a body and to the senses that I cannot exist without them? But I have persuaded myself that there is absolutaley nothing in the world: no sk, no earth, no minds, no bodies. Is it then the case that I too do not exist? But doubtless I did exist, if I persuaded myself of something." There are two possibilies: there is no God, then one is the author of oneself's thought ; there is a God, then how can he be sure that he exists? I understand this passage as, Descartes wants a solid point as a unshakable groung to build up all the coming things on it. He gives Archimedes as as example. "Archimedes sought but one firm and immovable point in order to move the entire earth from one place to another" (Med2) The example confuses me because Archimedes fixed point refers to the physical law from classical mechanics. But Descartes wants a solid ground as a firm reference frame for his truths. Why he gave Archimedes as as example?
In the Meditatioon 2, Descaretes continues with
- He already concludes that " I am, I exist".
- He asks "what I am?"
- He denies old answers to the question, like "I am a rational animal". Because the answer brings new concepts "rational" and "animal". H e doesn't want to deal with them because he searches new concepts which are more fundemantal and doesn't rely on shakable concepts he gained before meditation.
- He needs an answer to the question. He says, "...what came spontaneously and naturally into my thinking whenever I pondered what I was. Now it occured to me first that I had a face, hands, arms, and this entire mechanism of bodily members: the very same as are discerned in a corpse, and which I referred to by the name "body."...Descartes also links things with the soul.
- After some arguments, Descartes reaches the answer: "I am therefore precisely nothing but a thinking thing.
My question: How he gets the answer? He knows his body via his senses but sensations are body-dependent. Why he is not sure about his body but he is sure about he is thinking?
I guess René thought so because in his surroundings, he saw a few healers and doctors, otherwise we'd all be doctors and doctors, and later scientists and thinkers. He noticed that mankind cares more about the knowledge of the mind than the body :-)...