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Descartes 3

The proof of the existence of material things

Let us now turn to Meditation VI. Do material things exist? Well, they exist as the subject matter of pure mathematics -- and thus as objects of the understanding. That material things exist is also suggested by the imagination. The imagination is the application of the cognitive faculty to a body which is intimately present to it, and which therefore exists. (Rubin Pg. 40) The example of the pentagon and the chilliagon distinguish the intellect from the imagination. You should try the experiment of producing a mental image of a pentagon and then a chiliagon. The pentagon (with its five sides) has few enough sides so that we can form a distinct mental image of it. The chiliagon (with a thousand sides) has too many sides for us to form a distinct mental image of it. The idea of the pentagon, since it is a mental image, belongs to the imagination. On the other hand, while we have a clear and distinct idea of the chiliagon, that idea is not an image. The idea of the chiliagon is thus not of the imagination. Rather, it is a creature of the understanding. Descartes then argues that the imagination is not part of his essence. (Rubin Pg. 41) (Here we are getting a bit more about the self -- its essence is the understanding which grasps clear and distinct truths.) The nature of the imagination leads to an argument by inference to the best explanation for the existence of bodies. An inference to the best explanation says that if there is one explanation of a phenomena which is more likely than other competing explanations, this provides evidence that that explanation is true. Here Descartes says: "...it is easy to see how I get mental images if we suppose that my body exists. And, since I don't have in mind any other plausible explanation of my ability to have mental images, I conjecture that physical objects probably do exist. But this conjecture is only probable."

So, there were going to be three proofs for the existence of external bodies -- one from the understanding, one from imagination and one from the senses. We looked at the one from the understanding -- insofar as material bodies are objects of pure mathematics it is clear that they exist; one from the imagination -- insofar as I have these images, the best explanation of them is that they are caused by external objects; and finally Descartes turns to the senses, since they presumably have to do with the causes of these images. (Rubin Pg. 42)

The proof from the senses.

Now we will consider the proof from the senses. On Pg. 42 Descartes says, he will (1) "go back over all the things which I previously took to be perceived by the senses, and recognized to be true, and my reasons for thinking this." (2) Next he will set out his reasons for calling these things into doubt. and (3) He will consider what he should now believe about them.

(1) goes from 42 to 43 (2) from 43 to 44 and (3) from 44 to 46 in the paragraph which ends "...of pure mathematics.

In (1) Descartes begins with a list of things which he took to exist -- "heads, hands and feet, and other limbs making up my body, which I regarded as part of myself, or even perhaps as my whole self. Second this body is situated among other bodies, which could effect his body in various favorable or unfavorable ways, the favorable ones being indicated by pleasure and the unfavorable by pain. In addition, Descartes had sensations within him of hunger, thirst, and other such appetites, and physical propensities towards emotions, such as cheerfulness, sadness and anger. Considering the bodies outside him, he had tactile, and visual qualities which allowed him to distinguish, the sky the earth, the seas and other bodies.

He also thought that while his ideas of such things were the only immediate objects of sensory awareness, it was not unreasonable of him to think that these were caused by objects which were distinct from him. For the ideas came to him without his consent, and there could be no sensory awareness of an object unless it was present to him, nor could he avoid being aware of it if it was present. And these sensory ideas were much more alive and vivid that ideas which he took from his memory. He also had the notion that the ideas resembled these things. The senses preceded the use of reason and the ideas of reason were less vivid than those provided by the senses and derived from them. This led him to the empiricist axiom -- that there is nothing in the intellect which did not previously come from the senses. (Pg. 43) He also argues that he had good reason to hold that his body was special in that it was his. There were various things about his body which were taught to him by nature -- e.g. that a tugging sensation in the stomach should tell him that he should eat -- this had to be taught be nature because there is no obvious connection between cause and effect.

(2) Then on Pg. 43 (last paragraph) he gives the reasons why he came to doubt these things.

  1. Towers seen at a distance.
  2. Pain in limbs which are not there undermined his faith in the internal senses.
  3. The dream hypothesis -- Pg. 114
  4. The hypothesis that my nature might lead me to error.

(3) Now as 3 starts we get a surprise -- a proof for the real distinction of the mind and the body! What is this doing here?

Next we get the distinction between reason or understanding which is Descartes essence, and imagination and sensory perception which are faculties which belong to him, but which are not part of his essence. He distinguished between modes and substances of thinking and extended things.

On Pg. 45 we get the start of what we need for the proof from the senses of the existence of external objects. First Descartes distinguishes between a faculty of passive and a faculty of active perception. The passive faculty receives the images, while the active one produces them. He then considers the possibility that the active faculty of perception could be in him. He rejects this because it presupposes no intellectual act on his part, and the ideas in question are produced without his cooperation and often even against his will. So, the only alternative is that another substance distinct from him which contains either formally or eminently those ideas. How does Descartes proceed to argue from here that material substances must be the cause of his sensory ideas?

Pg. 46 Material objects do not exist exactly as I perceive them -- but one can distinguish here between those properties which are the objects of pure mathematics and those which are confused.

Pg. 46 What nature teaches me. I have a body. I am intermingled with my body -- here we get the not like a sailor in a ship metaphor.

Pg. 47 I am taught by nature that various other bodies exist in the vicinity of my body. These bodies possess differences corresponding to my differing perception, even if they do not resemble them.

Pg. 47 The things which I really was not taught by nature.

Pg. 48 This leads to rationalism and a rejection of empiricism.

Pg. 48-9 The issue of false judgment arises from the fact that Descartes has gotten to the point where he is discriminating between true and false beliefs about external objects. Thus at about 83 he is telling us that while feelings of heat and pain result from being near the fire, there is no reason for him to believe that there is anything in the fire which resembles heat and pain. (So we are wrong in thinking that ideas of sensible or secondary qualities resemble the qualities in the object). Another example is to conclude that the void exists because nothing is perceived between two objects. And the point of these example is that: "But I mistreat them as reliable touchstones for immediate judgment about the essential nature of bodies located outside of us; yet this is an area where they provide only obscure information." So, it appears, this is an application of the theory of error in MED. IV.

Now we turn to cases where nature presents objects for me to seek out and avoid, and the internal sensations where I have detected errors. First we have the poison case. Nature drives one towards what is normally a good thing. The fact that it is bad in this case would require greater knowledge than one has any reason to expect that one would have. So, in this case nature does not urge us towards something bad.

But there are cases where nature does urge us towards something, and it turns out to be the wrong course to take. Those who are ill may desire food which (presumably) they know is bad for them. Here we get the analogy with the clock, and the clock not telling time as much according to the laws of nature as something which does tell time. Here Descartes makes the distinction between two concepts of nature, one in the things themselves, another imposed by me.

Descartes then admits that in the case of the body suffering from dropsy, the term nature is not used as an extraneous label. In respect to the composite of mind and body, there is a real error of nature here, namely that it is thirsty at a time when drink will cause it harm. How is it, then Descartes asks, that the goodness of God prevents us from being deceived in this case? (Pg. 119)

Descartes first observation is to make the real distinction between the mind and the body again, this time in terms of the simplicity argument. Pp. 419-20

Descartes second observation that the mind is not affected by all parts of the body but by the brain or a small part of the brain (the pineal gland). He continues: "Every time this part of the brain is in a given state, it presents the same signals to the mind, even though the other parts of the body may be in a different condition at that time." This makes error possible.

Descartes third observation is that any part of the body which is moved by some other part some distance away, can be moved by an intermediate part.

Descartes fourth observation is that any given movement occurring in that part of the brain that immediately effects the mind produces just one corresponding sensation, and thus the best system that could be divised is that it produce that one sensation which of all possible sensations, is most especially and most frequently conducive to the preservation of man.


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