How did Descartes argue in the Sixth Meditation that mind and body are distinct substances? In what way does the Second Meditation contribute to his argument?

In Meditation VI, Descartes concludes two separate but related arguments that Mind and Body are distinct substances.   Both of these conclusions, however, are dependent on his earlier conclusion, established in Meditation II, that whatever he is, he is a substance whose essence is thinking.   He does not have enough knowledge available in Meditation II to immediately conclude that therefore Mind and Body are distinct.   That further conclusion must wait until he has re-established his connection with reality in Meditations IV and V, and can determine just what his body is in contrast with his mind.

In order to show how Descartes established that the mind and body are distinct substances, it will be simplest to follow his reasoning from Meditation II through to his conclusions in Meditation VI.

Having employed his "method of doubt,"that he describes in Meditation I, to call into question all other items and sources of knowledge -- particularly sensory awareness of the outside world, Descartes is left with one single assertion of which he is indubitably certain -- "I am, I exist".  

"[I]t must, in fine, be maintained, all things being maturely and carefully considered, that this proposition ‘I am, I exist', is necessarily true each time it is expressed by me, or conceived in my mind."
          Descartes / Vetch, Pg226 -- Meditation II

On this single premise, all of what follows in his reasoning is supposedly based.   But he goes on to inquire "What am I?"And he responds -- "But I do not yet know with sufficient clearness what I am, though assured that I am;"(ibid).  

Pursuing this question, Descartes proceeds to consider a number of possibilities.   For each alternative he considers, he recognizes that each presupposes some knowledge that he has, at this point, rejected as dubitable.   For example, when considering whether, in the Scholastic tradition, he is a "rational animal", he recognizes that such a description presupposes some meaning for "rational"and "animal".   Both concepts he argues depend on information from the senses that he has already rejected.   Similarly, he rejects the notion that he is a body, since all of the evidence he has of the existence of his body also comes from those same senses whose validity he has doubted. Finally, he reaches the conclusion that whatever else he might be, he is at least a mind -- a thinking thing.   Only that property of the "I"that exists is beyond doubt.   Only that property of the "I"that exists is not dependent on any other knowledge that has proven dubitable.

"I am therefore, precisely speaking, only a thinking thing, that is, a mind, understanding, or reason, terms whose signification was before unknown to me. I am, however, a real thing, and really existent; but what thing ? The answer was, a thinking thing."
          Descartes / Vetch, Pg 227 -- Meditation II

"It is, however, perfectly certain that the knowledge of my existence, thus precisely taken, is not dependent on things, the existence of which is as yet unknown to me; and consequently it is not dependent on any of the things I can feign in imagination."
          Descartes / Vetch, Pg 228 -- Meditation II

Thus, at the conclusion of the Second Meditation, Descartes has demonstrates that whatever it is that "I"am, I am at least a thinking thing that I can imagine (conceive of) as lacking everything else that is associated with that thinking, except thought.   This latter conclusion, that he can imagine himself existing as a thinking thing while lacking everything else that seems to be associated with his thinking, is crucial to his notions of "substance"and "essence".

The concepts of "Substance"and "Essence"were important concepts in the Aristotelian Scholastic philosophy in which Descartes was schooled (by the Jesuits at the Jesuit college of La Fleche, in Anjou).   According to Scholastic metaphysics, a "substance"is the most basic unit of existence.   And an "essence"was that quality or property that made something the type of thing that it is.   Every substance has its own unique and identifying essence.   From this educational background, it seemed so completely self-evident to Descartes that substances and essences were an integral part of reality, that it never occurred to him to subject the concepts of substance and essence to criticism in accordance with his program of systematic doubt.   Although he was prepared to doubt the existence of things that are observed by us (such as his body), he was not prepared to doubt the existence of substances and essences even though they are not directly observed by us.  

"As belonging to the class of things that are clearly apprehended, I recognize the following, viz, magnitude or extension in length, breadth, and depth; figure, which results from the termination of extension; situation, which bodies of diverse figures preserve with reference to each other; and motion or the change of situation; to which may be added substance, duration, and number."
          Descartes / Vetch, Pg 241 -- Meditation III

He held we must conclude that a substance is necessarily present wherever we encounter an "essential"property.   He uses the parable of the wax to demonstrate that the essential property of a substance is discernable not by the sense, but by the intellect alone.

"But what is the piece of wax that can be perceived only by the [understanding or] mind? It is certainly the same which I see, touch, imagine; and, in fine, it is the same which, from the beginning, I believed it to be. But (and this it is of moment to observe) the perception of it is neither an act of sight, of touch, nor of imagination, and never was either of these, though it might formerly seem so, but is simply an intuition of the mind, which may be imperfect and confused, as it formerly was, or very clear and distinct, as it is at present, according as the attention is more or less directed to the elements which it contains, and of which it is composed."
          Descartes / Vetch, Pg 231, - Meditation II

Descartes draws upon these unquestioned concepts of substance and essence to observe that this thinking thing that he is, must necessarily be a substance.

"For when I think that a stone is a substance, or a thing capable of existing of itself, and that I am likewise a substance, although I conceive that I am a thinking and non-extended thing, and that the stone, on the contrary, is extended and unconscious, there being thus the greatest diversity between the two concepts, yet these two ideas seem to have this in common that they both represent substances."         Vetch, Pg 242 -- Meditation III

Once he has identified himself (the "I"that is a thinking thing) as a substance, he draws upon his earlier conclusion that this substance "is not dependent on any of the things I can feign in imagination".   He can conceive of himself as lacking every property except thought.   Descartes therefore identifies that he is a substance whose essence is thought.

This much he accomplishes in Meditation II.   But this is not enough to complete his argument that Mind and Body are distinct substances.   Although he has concluded that the Mind is a substance whose essence is thought, it remains possible that the Body is also part (an "eminent"part) of that substance.   In order to eliminate that possibility, Descartes first has to prove the veracity of his "clear and distinct ideas"so that he can reconnect with the evidence of his senses.  

In Meditation III he demonstrates that God exists and can not possibly be a deceiver.   And in Meditation IV he completes the argument that as a result, whatever he perceives "clearly and distinctly"must be true..

"And certainly this can be no other than what I have   now explained: for as often as I so restrain my will within the limits of my knowledge, that it forms no judgment except regarding objects which are clearly and distinctly represented to it by the understanding, I can never be deceived; because every clear and distinct conception is doubtless something, and as such cannot owe its origin to nothing, but must of necessity have God for its author -- God, I say, who, as supremely perfect, cannot, without a contradiction, be the cause of any error; and consequently it is necessary to conclude that every such conception [or judgment] is true."
          Descartes / Vetch, Pg 257 -- Meditation IV

Having established the indubitable veracity of his "clear and distinct ideas", Descartes can then proceed to establish that the Body is also a substance with its own unique essence.   And hence, the Mind and the Body are two distinct substances.   He completes this reasoning in Meditation VI with two separate but mutually supporting arguments.

The first is based on the previously described doctrine of substance and essence.   Descartes claims that because he can form clear and distinct ideas of the mind and the body as distinct substances, each with their own essential uniquely identifying essence, each may exist without the other.   And hence they are necessarily two distinct substances.

"[T]herefore, merely because I know with certitude that I exist, and because, in the meantime, I do not observe that aught necessarily belongs to my nature or essence beyond my being a thinking thing, I rightly conclude that my essence consists only in my being a thinking thing [or a substance whose whole essence or nature is merely thinking]. And although I may, or rather, as I will shortly say, although I certainly do possess a body with which I am very closely conjoined; nevertheless, because, on the one hand, I have a clear and distinct idea of myself, in as far as I am only a thinking and unextended thing, and as, on the other hand, I possess a distinct idea of body, in as far as it is only an extended and unthinking thing, it is certain that I, [that is, my mind, by which I am what I am], is entirely and truly distinct from my body, and may exist without it."Vetch, Pg 269-70 -- Meditation VI

Slightly later in Meditation VI, Descartes reinforces this conclusion by arguing that the Mind is a substance that is indivisible, while the Body is a substance that is divisible.   Since a given substance cannot be both divisible and indivisible, he reasons that Mind and Body must be two distinct substances.

"[T]here is a vast difference between mind and body, in respect that body, from its nature, is always divisible, and that mind is entirely indivisible.   For in truth, when I consider the mind, that is, when I consider myself in so far only as I am a thinking thing, I can distinguish in myself no parts, but I very clearly discern that I am somewhat absolutely one and entire; and although the whole mind seems to be united to the whole body, yet, when a foot, an arm, or any other part is cut off, I am conscious that nothing has been taken from my mind; nor can the faculties of willing, perceiving, conceiving, etc., properly be called its parts, for it is the same mind that is exercised [all entire] in willing, in perceiving, and in conceiving, etc. But quite the opposite holds in corporeal or extended things; for I cannot imagine any one of them [how small so ever it may be], which I cannot easily sunder in thought, and which, therefore, I do not know to be divisible. This would be sufficient to teach me that the mind or soul of man is entirely different from the body, if I had not already been apprised of it on other grounds."
          Descartes / Vetch, Pg 276 -- Meditation VI

From this examination of Descartes reasoning, we can see that there are two contributions in Meditation II to the "proof"that the Mind and Body are distinct substances.   The first is the documentation of the doctrine of substances and essences, which he later uses to argue that substances with different essences must be different substances.   The second is the identification of his Mind as a substance whose essence (unique identifying property) is thought.   But it remains until Meditation VI for Descartes to demonstrate that the Body is also a substance with its own essential (unique identifying property) of extension.   This step in his reasoning must await Meditation VI because to identify the Body as a substance Descartes needs a way around his dismissal of his senses as dubitable.   If he cannot trust his sense, and all of the information he has about his Body is supplied by his senses, then he would have no basis from which to posit anything at all about the Body.   But once he has demonstrated that he can rely on the veracity of his "clear and distinct"ideas (in Meditations III and IV), he can employ this route to argue (in Meditation VI) that he has a "clear and distinct perception"of his Body.   Given the demonstrated indubitability of such a clear and distinct idea of the Body, he can then proceed to demonstrate that his Body is a substance with extension as its own essence.   With that final step accomplished, he can complete his argument that the Mind and the Body are clearly distinctly different substances.

References

Vetch, John (translator);   Descartes: The Method, Meditations, and Philosophy, Universal Classics Library, Walter Dunne Publisher, London, England, 1901. downloaded from The Online Library of Liberty, August 8, 2008. URL=<http://oll.libertyfund.org>

Williams, Bernard;   Descartes: The Project of Pure Enquiry, Penguin Books, New York, New York. 1990. ISBN 0-14-013840-4

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