Lecture Supplement on Descartes’ Meditations [1641] Part II

     Copyright © 2014 Bruce W. Hauptli

12. The Fourth Meditation:

Once Descartes has proved the existence of his deity, his problem undergoes a radical shift (or at least we may see it in this manner): prior to this point, he had to explain how the phenomenon of human knowledge was possible, but now (with a deity who is not a deceiver) his problem is to account for (or allow for) the phenomenon of error!  In philosophy of religion one of the core problems is the problem of evil.  Various religiously inclined philosophers offer theodicies which endeavor to prove that there is no inconsistency or contradiction between the existence of evil and the existence of an omni-powerful, omniscient, and omni-good deity.  In this Meditation, Descartes offers an argument akin to such theodicies in an effort to reconcile the existence of the sort of nondeceptive deity he thinks he has proven to exist and the existence of error.  In this Meditation he shows not only how we fall into error, but how we can avoid it and obtain truth about other things (than our existence and the existence of the deity). 

81 Recapitulation. 

"And from the fact that this idea is in me, or that I who have this idea exist, I draw the obvious conclusion that God also exists, and that my existence depends entirely upon him at each moment.  This conclusion is so obvious that I am absolutely confident that the human mind can know nothing more evident or more certain.1  And now I seem to see a way by which I might progress from this contemplation of the true God, in whom, namely, are hidden all the treasures of the sciences and wisdom, to knowledge of other things." 

His deity does not deceive us; for it is imperfect to deceive; and we have a faculty of judgment which we may use to distinguish truth from error—a faculty which we received from his deity.2 He would not give us a faculty which would lead us astray (if we use it correctly). 

-82 "...I make mistakes because the faculty of judging the truth, which I got from God, is not, in my case infinite."

-"No doubt God could have created me such that I never erred. No doubt, again, God always wills what is best. Is it then better that I should be in error than not?"

-82-83 "...whenever we ask whether the works of God are perfect, we should keep in view not simply some one creature in isolation from the rest, but the universe as a whole. For perhaps something might rightfully appear imperfect if it were all by itself, and yet be most perfect, to the extent that it has the status of a part in the universe." 

83 "...[my] errors depend on the simultaneous occurrence of two causes: the faculty of knowing that is in me and the faculty of choosing, that is, the free choice of the will, in other words, simultaneously on the intellect and will." 
-84 Descartes contends that the faculty of understanding is limited in us (but not in his deity), as are the faculties of memory, imagination, perception, etc. (and, again, in his deity, he holds, these faculties are infinite).  On the other hand, our faculty of willing resembles that of the deity—it is (nearly boundless). 

-84 "...the power of willing, which I got from God, is not, taken by itself, the cause of my errors, for it is most ample as well as perfect in its kind.  Nor is my power of understanding the cause of my errors. For since I got my power of understanding from God, whatever I understand I doubtless understand rightly, and it is impossible for me to be deceived in this.  What then is the source of my errors? T hey are owing simply to the fact that, since the will extends further than the intellect, I do not contain the will within the same boundaries; rather, I also extend it to things I do not understand.  Because the will is indifferent in regard to such matters, it easily turns away from the true and the good; and in this way I am deceived and I sin." 

--He clearly presupposes some form of "epistemic voluntarism here (that our beliefs are, to some extent, under our control.  He also believes we have "epistemic responsibility"—that we are, to some extent, responsible for our beliefs. 

85 "...it is manifest by the light of nature that a perception on the part of the intellect must always precede a determination of the will." 
-86 His deity does not cause the errors, they are the result of privation (the fact that we were created with finite understanding [and infinite will]). 

-"...I should never judge anything that I do not clearly and distinctly understand." 

-87 "...as often as I restrain my will when I make judgments, so that it extends only to those matters that the intellect clearly and distinctly discloses to it, it plainly cannot happen that I err.  For every clear and distinct perception is surely something, and hence it cannot come from nothing.  On the contrary, it must necessarily have God for its author: God, I say, that supremely perfect being to whom it is repugnant to be a deceiver....Therefore the perception is most assuredly true." 

13. The Fifth Meditation:

87 Descartes says that "...nothing seems more pressing than that...I see whether anything certain is to be had concerning material things."  But before going on to deal with our knowledge of material things (the central topic of the Sixth Meditation), he discusses our knowledge of mathematics and of his deity (again).  This discussion is important because it provides him with a first and general opportunity for indicating how one proceeds from clear and distinct ideas to the world, and because it indicates something about the world itself independent of our thoughts about it. 

(A) Descartes' Argument for his Knowledge of Mathematical Objects:

88 "...I find within me countless ideas of certain things, that, even if they do not exist anywhere outside me, still cannot be said to be nothing. And although, in a sense, I think them at will, nevertheless they are not something I have fabricated; rather they have their own true and immutable natures. For example, when I imagine a triangle even if perhaps no such figure exists outside my thought anywhere in the world and never has, the triangle still has a certain determinate nature, essence, or form which is unchangeable and eternal, which I did not fabricate, and which does not depend on my mind." 
-What does this paragraph say?  Draw a triangle on the board.  Is it a triangle?3  Where are triangles?  Are they real

-88 Some mathematical truths are listed and he says that "all these properties are patently true because I know them clearly, and thus they are something and not merely nothing. For it is obvious that 

[1] whatever is true is something [truths are about something],

[2] all that I know clearly [and distinctly] is true,

[3] it follows that all that I clearly and distinctly perceive to belong to that thing really does belong to it,

[4] therefore (from 1, 3, and the example of the triangle), mathematical things are (that is, they exist]. 

(B) Descartes' Ontological Argument for his Deity's Existence:

At this point, it appears, there is a gigantic switch of topic, both in this Meditation, and in the Meditations in general.  We move to an ontological proof for the existence of a deity.  Is this such a major shift however?  The certainty Descartes arrives at in his earlier Meditations, as he himself admits, depends upon the nonexistence of an evil genius and upon the existence of a deity. H is proofs hang in limbo as long as they begin with that which is dependent and endeavor to move forward.  He must "begin" [ontologically speaking] with that which is wholly independent, and show (with certainty) that this starting point is secure.  The goal of the following proof is to provide him with an "ultimately secure" beginning point—to provide Descartes with the metaphysical ground upon which his epistemological conclusions may stand. 

Throughout the Meditations, Descartes has been concerned with metaphysics.  He has asserted that he is a substance; drawn a distinction between mental and physical substances (and between finite and infinite ones); discussed the status of attributes; offered a causal principle; discussed the objective reality (representational capacity of ideas); pointed out that true ideas must represent something real; and, now, discussed the status of mathematical truths.  Clearly, he is vitally concerned with metaphysics, but he has not discussed the topic directly on its own.  He has been discussing and developing his topics according to an "epistemological order" (following what commentators call "the order of ideas"); and he now pauses to discuss to discuss things according to a "metaphysical order" (what commentators call "the order of things [or being]").  This means, given his orientation, that he must start with the foundation of everything which exists, and this is, of course, his deity.  Thus we have a second, and very different, argument for the existence of his deity--one which doesn't begin from the self and work outward to the deity.  Instead, it begins with the deity itself and shows that it, by its very nature, must [necessarily] exist.3a 

88 1. Concept of a supremely perfect being [the essence of his deity]. 
-Discuss essences!

89 2. He has been accustomed in every other matter to distinguish between existence and essence; and so he believes that the existence can be separated from the essence of his deity, and that, thus, his deity may be conceived as not actually existing.

3. But this case is different: "...the existence can no more be separated from the essence of God, than the idea of a mountain from that of a valley, or the equality of its three angles to two right angles, from the essence of a triangle; so that it is not less impossible to conceive a God, that is, a being supremely perfect, to whom existence is awanting, or who is devoid of certain perfection, than to conceive a mountain without a valley."

4. Therefore, his deity exists necessarily. Note that there is a difference between concluding that "it is necessary to conclude that God exists" [Meditation III] and concluding that "necessarily, God exists". (See long citation on pp. 90-91). 

-Difference of this case and that of mountain-valley. 

90 Summation of the proof...his deity is eternal and unique:

-"Next, I cannot understand how there could be two or more Gods of this kind.  Again, once I have asserted that God now exists, I plainly see that it is necessary that he has existed from eternity and will endure for eternity."4 

--Why is it necessary that his deity be unique—that there be only one of such things?  Couldn’t “what’s real is rational, and what’s rational is real” be true if there are several such beings?  The need for it to be a universe! 

--Why is it necessary that his deity is "eternal?"  Because if it were not, it would be caused, and it would have an end, and neither of these occurrences would be compatible with necessary existence. 

90-91 "...the things that fully convince me are those that I clearly and distinctly perceive.  And although some of these things I thus perceive are obvious to everyone, while others are discovered only by those who look more closely and inquire carefully, nevertheless, once they have been discovered, they are considered no less certain than the others. For example, in the case of a right triangle, although it is not so readily apparent that the square of the hypotenuse is equal to the sum of the square of the other two sides as it is that the hypotenuse is opposite the largest angle, nevertheless, once the former has been ascertained, it is no less believed.  However, as far as God is concerned, if I were not overwhelmed by prejudices and if the images of sensible things were not besieging my thought from all directions, I would certainly acknowledge nothing sooner or more easily than him.  For what, in and of itself, is more manifest than that a supreme being exists, that is, that God, to whose essence alone existence belongs, exists?" 

-91 "...I now am just as certain about this as I am about everything else that seems most certain. Moreover, I observe also that certitude about other things is so dependent on this, that without it nothing can ever be perfectly known."5 

His deity guarantees that past clear and distinct ideas may be relied upon:"...even if I no longer attend to the reasons leading me to judge this to be true, so long as I merely recall that I did clearly and distinctly observe it, no counter-argument can be brought forward that might force me to doubt it. On the contrary, I have a true and certain knowledge of it." 

-92 "...even if I were dreaming, if anything is evident to my intellect, then it is entirely true.

And thus I see plainly that the certainty and truth of every science depends exclusively upon the knowledge of the true God, to the extend that, prior to my becoming aware of him, I was incapable of achieving perfect knowledge about anything else.  But now it is possible for me to achieve full and certain knowledge about countless things...." 

So, at the end of this Meditation we each have knowledge of ourself, a deity, mathematics, and the "guarantee" that clear and distinct ideas serve as a valid criterion of truth.  For Descartes this deity serves as the validator, and clarity and distinctness serves as the criterion of knowledge (about self, deity, and world)—of course, the deity "validates" the criterion. 

William Alston offers an excellent critical treatment of Descartes' version of the ontological argument in his "The Ontological Argument Revisited."6 

14. The Sixth Meditation:

The main problem in this Meditation is: "Does material substance exist?" or, perhaps better, "Can we know that material substance exists?"  To provide an answer, he offers two proofs: an initial, or tentative one; and a second, more careful, one.  After these proofs, Descartes distinguishes what we are "taught by nature" from what is known via the "natural light," and discusses the composite of the body and mind which we call the "self"—what I will call the "body-mind." 

While it is natural to focus attention on the proof(s) regarding material substance, it is profitable to briefly hold on to what may well be a non-standard reading, and consider that this Meditation might well be considered to be primarily about the science of "psychology" rather than about the "physical sciences."  Thus far we have seen Descartes claim that the self exists, that it is, essentially, a thinking thing, that it has several faculties (notably understanding and willing), that "agent-causation" applies to it (though the extent of this is still under scrutiny), and that it is finite.  In this Meditation we learn much more about the self, or mind--(and such learning, surely, falls under the general banner [now] of psychology). I n this Meditation we learn about two other "faculties:" imagination and perception, we learn that it is in our perceptions of pleasure and pain (that is, certain of our "ideas") that we get our best knowledge about the "external world" (which causes these ideas as they can not be caused either by us or by his deity), and that these "perceptions" are fallible, and caused by "other things." As is the case with the wax experiment (though Descartes does not note this here as he does there), what we learn with the greatest certainty is things about our minds, and what is less (far, far less) "certain" is any understanding of "bodies." 

(A) He begins with a "tentative" proof which holds that our faculty of imagination shows that material things probably exist. 

92 "...imagination...appears to be a certain application of the knowing faculty to a body immediately present to it, which therefore exists." 
-92-93 Distinguish understanding and imagining triangles, on the one hand, and understanding and imagining "chiligons" (one thousand sided figures), on the other.  Consider also the case of pentagons.  We can see that [94] "...a particular sort of effort on the part of the mind is necessary in order to imagine, one that I do not employ in order to understand." 

-93 "...this power of imagination...is not required for my own essence...." 

-Thus, there is something different from the mind upon which this faculty depends. 

-"...were a body to exist to which a mind is so joined that it may apply itself in order, as it were, to look at it any time it wishes, it could happen that it is by means of this body that I imagine corporeal things.  As a result, this mode of thinking may differ from pure intellection only in the sense that the mind, when it understands, in a sense turns toward itself and looks at one of the ideas that are in it; whereas when it imagines, it turns toward the body, and intuits in the body something that conforms to an idea either understood by the mind or perceived by sense." 

"To be sure, I easily understand that the imagination can be actualized in this way, provided a body does exist. And since I can think of no other way of explaining imagination that is equally appropriate, I make a probable conjecture from this that a body exists." 

-Note the more reserved conclusion—compare his claim here with his knowledge of self, his deity, mathematics, or (again) of his deity! 

-Note, also that there is here the kernel of the contemporary epistemological notion of inference to the best available explanation.7 

Problem: we might imagine other explanations [which don't seem to require bodies], and so we need a more careful consideration and a better argument. 

(B) The "second," and more careful, proof: removing the reasons for doubting:
94 "...I will review...all the things I previously believed to be true because I had perceived them by means of the senses and the causes I had for thinking this.  Next I will assess the causes why I called them into doubt.  Finally, I will consider what I must now believe about these things." 

1. What he believed and why—that his "sensory" ideas truly represented "things:"

-"..I sensed that I had a head, hands, feet, and other members that comprised this body which I viewed as part of me, or perhaps, even as the whole of me. I sensed that this body was found among many other bodies...." 

-"...these ideas came upon me utterly without my consent, to the extent that, wish as I may, I could not sense any object unless it was present to a sense organ.  Nor could I fail to sense it when it was present. And since the ideas perceived by sense were much more vivid and explicit and even, in their own way, more distinct that any of those that I deliberately and knowingly formed through mediation or that I found impressed on my memory, it seemed impossible that they came from myself.  Thus the remaining alternative was that they came from other things.  Since I had no knowledge of such things except from those same ideas themselves, I could not help entertaining the thought that they were similar to those ideas." 

95 2. Why he doubted:
-Perceptual problems: towers from afar; external and internal senses subject to error—phantom limb pains; dreaming; and the evil genius. 

-95-96 "As to arguments that used to convince me of the truth of sensible things, I found no difficulty responding to them.  For since I seemed driven by nature toward many things about which reason tried to dissuade me, I did not think that what I was taught by nature deserved much credence.  And even though the perceptions of the senses did not depend on my will, I did not think that we must therefore conclude that they carry from things distinct from me, since perhaps there is some faculty in me, as yet unknown to me, that produces these perceptions." 

96 3. Why these doubts should now be rejected:

"But now, having begun to have a better knowledge of myself and the author of my origin, I am of the opinion that I must not rashly admit everything that I seem to derive from my senses; but neither, for that matter, should I call everything into doubt." 

-(i) "First, I know that all the things that I clearly and distinctly understand can be made by God such as I understand them.  For this reason, my ability clearly and distinctly to understand one thing without another suffices to make me certain that the one thing is different from the other, since they can be separated from each other, at least by God." 

-(ii) Mind and body are (clearly and distinctly) separable—body is no part of my essence. 

-(iii) The faculties of imagining and perceiving are distinct from me—as are the faculties of motion and taking on shape. 

-96-97 (iv) "But it is clear that these faculties, if in fact they exist, must be in a corporeal or extended substance, not in a substance endowed with understanding.  For some extension is contained in a clear and distinct concept of them, though certainly not any understanding.  Now there clearly is in me a passive faculty of sensing, that is, a faculty for receiving and knowing the ideas of sensible things; but I could not use it unless there also existed, either in me or in something else, a certain active faculty of producing or bringing about these ideas." 

-97 (v) This active faculty can not be in me [that is, in me essentially and insofar as I am a thinking thing]—it does not presuppose thought and the ideas in question are frequently caused against my will. 

-(vi) "Therefore the only alternative is that it is in some substance different from me containing either formally or eminently all the reality that exists objectively in the ideas produced by that faculty....Hence, this substance is either a body, that is, a corporeal nature, which contains formally all that is contained objectively in the ideas, or else it is God, or some other creature more noble than a body, which contains eminently all that is contained objectively in the ideas." 

-(vii) His deity can not be the substance in which this active faculty inheres (because he is not a deceiver) and, therefore, it must exist in bodies

--"For since God has given me no faculty whatsoever for making this determination, but instead has given me a great inclination to believe these ideas issue from corporeal things, I fail to see how God could be understood not to be a deceiver, if these ideas were to issue from a source other than corporeal things. And consequently corporeal things exist."
(C). We are taught "by nature" about bodies (especially about the "body-mind"):
97 Regarding "particular" claims regarding corporeal bodies (that the sun is of such and such a size, what light, sound, and pain are, etc.), we can reach some understanding: in such cases, we are "taught by nature:"
-In her "Cartesian Passions: The Ultimate Incoherence," Marjorie Grene maintains that for Descartes we know the mind and body: "...by natural light, but [the union of the mind and body] only through that `natural impulse' which Descartes has warned us in the Third Meditation has so often misled us, not only in the search for truth but in the choice of goods.  It seems predestined, therefore, that the interrelations of mind and body, the events of our lives as united minds and bodies, that our human histories, in short, and therefore our human being, can be comprehended only in this relatively obscure, non-scientific and instinctive rather than in that luminous, intellectual way.  As Descartes emphasizes, the union of the body and mind is accessible to us, not through science, but through the experience of ordinary life; and it is precisely the experience of everyday life that has been definitively undermined in the programmatic doubt of Meditation One, followed by the demonstration in Meditation Two of its incurable confusion."8 

Descartes discusses, in order of "clarity," a number of things which are disclosed "by nature."  First, "there is nothing that this nature teaches me more explicitly than that I have a body that is ill-disposed when I feel pain, that needs food and drink when I suffer hunger or thirst, and the like.  Therefore, I should not doubt that there is some truth in this." 

98 Second, "by means of these sensations of pain, hunger, thirst and so on, nature also teaches not merely that I am present to my body in the way a sailor is present in a ship, but that I am most tightly joined and, so to speak, commingled with it, so much so that I and the body constitute one single thing." 

Thirdly, nature teaches me that my body is surrounded by other bodies. 

Finally, "...there are many other things that I seem to have been taught by nature; nevertheless it was not really nature that taught them to me but a certain habit of making reckless judgments.  And thus it could easily happen that these judgments are false....to ensure that my perceptions in this matter are sufficiently distinct, I ought to define more precisely what exactly I mean when I say that I am "taught something by nature." 

-What I am "taught by nature" is not the same thing as what arises via the "natural light!"  We are "taught by nature" to:
--99 avoid things that produce sensations of pain,

--pursue things that produce sensations of pleasure,

--"but it does not appear that nature teaches us to conclude anything besides these things, from these sense perceptions unless the intellect has first conducted its own inquiry regarding things external to us.  For it seems to belong exclusively to the mind, and not to the composite of mind and body, to know the truth in these matters." 

-"...I use the perceptions of the senses (which are properly given by nature only for signifying to the mind what things are useful or harmful to the composite of which it is a part, and to the extent that they are clear and distinct enough) as reliable rules for immediately discerning what is the essence of bodies located outside us.  Yet they signify nothing about that except quite obscurely and confusedly." 
--I may be mistaken even about pleasure and pain—pleasant taste and poison, the ill who desire food or drink which will make them worse!  This means that this "knowledge" is not the same sort of knowledge as that which he gets when he confines himself to the mental domain! 

-100 While a poorly designed clock doesn't tell time well, we can not conclude that our bodies have been poorly designed (that they don't "tell well" the nature of external things).  To see that his deity is not to be "blamed" for misconstruction, we must better understand the mind, body, and body-mind. 

100-101 Bodies are divisible, minds are indivisible,

-101 the mind is not affected by the whole body, but only by the brain,

-the body is a composite such that "remote" movements require "intermediate" movements, and the sensory "results" could be achieved by the intermediate movements only (pain "in foot" requires transmission through intermediates, which could alone cause the pain). 

-102 "...notwithstanding the immense goodness of God, the nature of man, insofar as it is composed of mind and body, cannot help being sometimes mistaken.  For if some cause, not in the foot but in some other part through which the nerves extend from the foot to the brain, or perhaps even in the brain itself, were to produce the same motion that would normally be produced by a badly injured foot, the pain will be felt as if it were in the foot, and the senses will naturally be deceived."  Ditto, of course, for "dryness of the throat" in an ill individual. 

-103 "...I know that all the senses set forth what is true more frequently than what is false regarding what concerns the welfare of the body.  Moreover, I can nearly always make use of several of them to examine the same thing.  Furthermore, I can use my memory, which connects current happenings with past ones, and my intellect, which now has examined all the causes of error.  Hence I should no longer fear that those things that are daily shown me by the senses are false.  On the contrary, the hyperbolic doubts of the last few days ought to be rejected as ludicrous." 

--"This goes especially for the chief reason for doubting, which dealt with my failure to distinguish being asleep from being awake.  For I now notice that there is a considerable difference between these two; dreams are never joined by the memory with the other actions of life, as is the case with those actions that occur when one is awake....and when I connect my perception of them with the rest of my life, I am clearly certain that these perceptions have happened to me not while I was dreaming but while I was awake.  Nor ought I have even the least doubt regarding the truth of these things, if, having mustered all the senses, in addition to my memory and my intellect, in order to examine them, nothing is passed on to me by one of those sources that conflicts with the others.  For from the fact that God is no deceiver, it follows that I am in no way mistaken in these matters.  But because the need to get things done does not always permit us the leisure for such a careful inquiry, we must confess that the life of man is apt to commit errors regarding particular things, and we must acknowledge the infirmity of our nature." 
(End of the Meditations)

15. Additional Points About the Body-Mind:

In his The Passions of the Soul [1649], Descartes maintains that it is in the pineal gland that the mind and body interact:

...although the soul is joined to the whole body, there is yet in that a certain part in which it exercises its functions more particularly than in all the others; and it is usually believed that this part is the brain, or possibly the heart: the brain, because it is apparently in it that we experience the passions.  But, in examining the matter with care, it seems as though I had clearly ascertained that the part of the body in which the soul exercises its functions immediately is in nowise the heart, nor the whole of the brain, but merely the most inward of all its parts, to wit, a certain very small gland which is situated in the middle of its substance and so suspended above the duct whereby the animal spirits in its anterior cavities have communication with those in the posterior, that the slightest movements which take place it may alter very greatly the course of these spirits; and reciprocally that the smallest changes which occur in the course of the spirits may do much to change the movements of this gland.9 

Let us then conceive here that the soul has its principal seat in the little gland which exists in the middle of the brain, from whence it radiates forth through all the remainder of the body by means of the animal spirits, nerves, and even the blood, which, participating in the impressions of the spirits, can carry them by the arteries into all the members.  And reconciling what has been said above about the machine of our body, i.e. that the little filaments of our nerves are so distributed in all its parts, that on the occasion of the diverse movements which are there excited by sensible objects, they open in diverse ways the pores of the brain, which causes the animal spirits contained in these cavities to enter in diverse ways into the muscles, by which means they can move the members in all the different ways in which they are capable of being moved...let us here add that the small gland which is the main seat of the soul is so suspended between the cavities which contain the spirits that it can be moved by them in as many different ways as there are sensible diversities in the object, but that it may also be moved in diverse ways by the soul, whose nature is such that it receives in itself as many diverse impressions....10 

This may sound utterly foolish to anyone who has had even a rudimentary acquaintance with "human anatomy," but it is important to note that William Harvey's A Disquisition on the Circulation of the Blood (which proved that blood was pumped through the body by the heart) was published in 1615, that Descartes did ground-breaking anatomical research (discovering and showing that muscles worked in pairs with one "retracting" and the other "relaxing" in each movement), and that Benjamin Franklin's Experiments and Observations on Electricity was still more than a hundred years in the future [1751] (and thus the mechanism by which the nerves worked could not be clear).  Descartes' account of the interaction of mind and body, then would not have seemed as obviously false then as it does now.  Nonetheless, his account of the union of the mind and body immediately raises the question as to who these radically different sorts of substance could interact.  This problem is with us today! 

At the end of the Meditations we have mind (both ours and that of the deity), extended substance, and the body-mind composite.  Of the first two (as the citation from Grene above clarifies) we can have certain knowledge, but of the third our understanding is far from clear.  Descartes' correspondence with Princess Elizabeth of Bohemia [1618-1680] helps point to the problems which this third "primitive notion" raise for Descartes.11  In his Principles of Philosophy, which he dedicated to the Princess, Descartes endeavors to clarify the status of claims about the body-mind as follows:

there remain our sensations, affections and appetites, as to which we may likewise have a clear knowledge, if we take care to include in the judgments we form of them that only which we know to be precisely contained in our perception of them, and of which we are intimately conscious.  It is, however, most difficult to observe this condition, in regard to our senses at least, because we, everyone of us, have judged from our youth up that all things of which we have been accustomed to have sensation have had an existence outside our thoughts, and that they have been entirely similar to the sensation, that is the idea which we have formed of them.  Thus, when for example, we perceived a certain colour, we thought that we saw something which existed outside us and which clearly resembled the idea of colour which we then experienced in ourselves, and from the habit of judging in this way we seemed to see this so clearly and distinctly as to be convinced that it is certain and indubitable.12 
Of course, in such judgments we can easily be quite mistaken, and thus there can be little indubitable knowledge (if any) in regard to the union of the body and mind (or in regard to extended substance).  Indeed, there is so little knowledge that two "famous philosophical problems" arise at this point (in addition to the problem of how the two distinct substances could possibly interact which was mentioned above):
A. The Problem of the External World: how can we, beginning only with our sensations, ideas, and concepts, prove anything about particular "external objects" (even that they exist)? and

B. The Problem of Other Minds: how can we, beginning with our own personal sensations, ideas, and concepts, prove that other conscious mental beings exist?  Note that in the above citation from the Principles, Descartes talks about "us," but when we follow the "order of ideas" in the Meditations, the best we get in Meditation Six is that one's own union of mind and body exists!  The argument he offers, and the cautions he offers against making indubitable claims about bodies, preclude, it would seem, knowledge about the existence of other individuals. 

16. The "Idea" Idea: More on the Mind-Body Distinction:

In an extremely interesting article, Wallace Matson explains an important difference between the "modern" and the "ancient" world-views.  He maintains that the two periods have radically different ways of looking at the separation of the mind and body:

the Greeks did not lack a concept of mind, even of a mind separable from the body.  But from Homer to Aristotle, the line between mind and body, when drawn at all, was drawn so as to put the processes of sense perception on the body side.  That is one reason why the Greeks had no mind-body problem.  Another is that it is difficult, almost impossible, to translate such a sentence as "What is the relation of sensation to mind (or soul)?" into Greek.  The difficulty is in finding a Greek equivalent for "sensation" in the sense philosophers make it bear...."Sensation" was introduced into philosophy precisely to make it possible to speak of a conscious state without committing oneself as to the nature or even existence of external stimuli. 
Commenting upon this passage, Richard Rorty maintains that:
...in Greek there is no way to divide "conscious states," or "states of consciousness"—events in an inner life—from events in an "external world."   Descartes, on the other hand, used "thought" to cover doubting, understanding, affirming, denying, willing, refusing, imagining, and feeling, and said that even if I dream that I see light "properly speaking this in me is called feeling, and used in this precise sense that is no other thing than thinking." Once Descartes had entrenched this way of speaking it was possible for Locke to use "idea" in a way which has no Greek equivalent at all, as meaning "whatsoever is the object of the understanding when a man thinks" or "every immediate object of the mind in thinking."  As [Anthony] Kenny puts it, the modern use of the word idea derives through Locke from Descartes, "and Descartes was consciously giving it a new sense...it was a new departure to use it systematically for the contents of a human mind."  More important, there had been no term, even of philosophical art, in the Greek and medieval traditions coextensive with the Descartes-Locke use of "idea."  Nor had there been the conception of the human mind as an inner space in which both pains and clear and distinct ideas passed in review before a single Inner Eye....the seventeenth century took it seriously enough to permit it to pose the problem of the veil of ideas, the problem which made epistemology central to philosophy. 
Notes: (click on note number to return to the text for the note)

1 Note that while he has argued that we must conclude that an infinite and perfect deity exists (and is not a deceiver), his proof in the previous "Meditation" did not establish that his deity causes him, others, or other things.  While being all-powerful would, of course, allow for this, it doesn't require it—omnipotence is an ability, but it might not be exercised.  Here, then, there is the danger that Descartes is (simply) assuming that his deity is (the only) causally efficacious force (ultimately speaking). 

2 Note the difference between a picture, or representation of a triangle, and a triangle!  3 Note that from the fact that "he can not understand how there could not be two" deities of the identified type, it clearly does not follow that is only one (let alone follow with any sort of necessity).  Were he to say that he clearly and distinctly "perceives" (conceives) that there is only one, then, at least within his world-view, this might follow, but he doesn't make even that claim here. 

3a For another class I have a lecture supplement on Anselm's ontological argument, and students may find this helpful in both understanding Descartes' argument, and in contrasting Anselm's medieval orientation with Descartes' modern one. 

4 Note the relevance of this passage to the issue of circularity which arose in Meditation Three

5 William Alston, "The Ontological Argument Revisited," in Descartes: A Collection of Critical Essays, ed. Willis Doney (Garden City: Anchor, 1967), pp. 278-302.  The essay originally appeared in The Philosophical Review v. 69 (1960), pp. 452-474. 

6 Cf., Gilbert Harman, Thought (Princeton: Princeton U.P., 1973).  His discussion on pp. 130-141, and 155-172 provides a concise account of this orientation.  

7 Marjorie Grene, "Cartesian Passions: The Ultimate Incoherence," in her Descartes (Minneapolis: Univ. of Minnesota, 1985), pp. 23-52, p. 33. 

8 Descartes, The Passions of the Soul [1649] I 31, in The Philosophical Works of Descartes v. 1, op. cit., pp. 347-348. 

9 Cf., Descartes' letter to Elizabeth on May 21, 1643.  It can be found in Women Philosophers of the Early Modern Period, ed. Margaret Atherton (Indianapolis: Hackett, 1994), pp. 12-15.  Cf., p. 13 for his discussion of the mind, body and union of the two as three "primitive notions."   See also my lecture supplement  “The Descartes-Elizabeth Correspondence.”   

10 Descartes, Principles of Philosophy, I 66, op. cit., p. 247. 

11 Wallace Matson, "Why Isn't the Mind-Body Problem Ancient?", in Mind, Matter and Method: Essays in the Philosophy of Science in Honor of Herbert Feigel, eds. Paul Feyerabend and Grover Maxwell (Minneapolis: Univ. of Minnesota, 1966), pp. 92-102, p. 101. 

12 Richard Rorty, Philosophy and The Mirror of Nature (Princeton: Princeton U.P., 1979), pp. 47-51.  Rorty's citations (the passage has over two pages of citations and footnotes associated with it) are to: Descartes' Meditations; Locke's Essay Concerning Human Understanding, and "Second Letter to the Bishop of Worcester;" and to Anthony Kenny's "Descartes on Ideas" which is included in Descartes: A Collection of Critical Essays, op. cit., pp. 227-249, p. 226. 

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Last revised on: 09/09/2014