Illustration showing the vortex of the sun S surrounded by vortices of other stars.
Descartes' chief philosophical works were the Meditations on First Philosophy and the Principles of Philosophy. In the Principles, he provides a metaphor for the relationship of the parts of philosophy. At the root is metaphysics, the most general part; the trunk is physics, the general science of material beings, and the branches are the specific applied sciences of medicine, mechanics and ethics. Thus in that work, Descartes tried to work out a comprehensive philosophy starting with metaphysics and ending with mechanics. In the Meditations, he attempted to justify his metaphysics through consideration of the question of what we can know.
One of the most notable aspects of Descartes's philosophy was the attention he paid to methodology. He believed that before one could conduct proper scientific investigation, one must first know how claims about nature can be justified. There are four different routes to the understanding of the natural world, and each has its own type of validation.
In Principles of Philosophy, Part II, Section 64, Descartes wrote: "I openly acknowledge that I know of no kind of material substance other than that which can be divided, shaped, and moved in every possible way, and which Geometers call quantity and take as the object of their demonstractions. And [I also acknowledge] that there is absolutely nothing to investigate about this substance except thosed divisions, shapes and movements; and that nothing concerning these can be accepted as true unless it is deduced from common notions, whose truth we cannot doubt, with such certainty a that it must be considered as a Mathematical demonstration. And because all Natural Phenomena can thus be explained, as will appear in what follows; I think that no other principles of Physics should be accepted, or even desired."
Descartes here considerably overstates the role of mathematics in physical explanation. Geometry only dictates the static properties of objects and is silent about their dynamical interactions. Suppose a sphere A, one foot in diameter and moving at a speed of one foot per second, were to collide with another sphere B of the same size, but at rest. What would result from the collision? There are any number of possibilities consistent with geometry: Would A continue to move, and in which direction? Would B continue at rest? Do we have enough information even to decide these questions?
The conservation law was fatally flawed, however. Motion is measured by what can be called speed, a function of distance and time. It leaves out of account the direction of motion. (Speed is a "scalar" quantity, while directed speed, or velocity, is a "vector" quantity.) Speed is not conserved in some collisions of bodies, as other scientists of the time showed. (Huyghens and Leibniz were two who pointed this out.) Furthermore, Newton held that even velocity is not conserved in some collisions. When two perfectly hard bodies (that is, bodies which cannot become compressed) collide when moving on a straight line toward each other at the same speed, they stop. This issue was a matter of debate between Leibniz and the followers of Newton.
The most important suppositions concern the behavior of insensible particles of matter. Like Galileo, Descartes held that these particles really possess a few "primary" qualities. "Apart from size, figure, and motion, the varieties of which I have explained as they are in each body, nothing located outside us is observed except light, color, odor, taste, sound, and tactile qualities; whch I have now demonstrated are nothing in the objects other than, or at least are perceived by us as nothing other than, cerain dispositions of size, figure and motion of bodies" (Principles, Part IV, Section 199).
Imperceptible particles are nothing more than small extended bodies: "Judging them all to be made of the same material, I believe that each one could be redivided in an infinity of ways, and that they differ among themselves only as pebbles of many different shapes would differ, had they been cut from the same rock" (Meteorology, First Discourse). To explain the cohesion of earthy bodies, Descartes supposed that they are made of roughly-shaped particles that intertwine easily, as opposed to the long, slippery, smooth components of water. Filling the gaps between these larger particles are very fine pieces of matter which transmit light. Like Aristotle and unlike the ancient atomists, Descartes did not suppose the existence of a void.
That all of space is filled was of special significance for Descartes, for it allowed him to explain the motions of the heavenly bodies by means of a supposition based on a mechanical system. The heavens consist of fluid vortices, like the eddies in a river where the water doubles back upstream. Each star, including the sun, is the center of its own vortex. Just as a piece of straw is carried around in a circle, a planet is carried around the center of the vortex. And just as objects closer to the center of the vortex move faster than those on the edge, the innermost planets move faster than those farther from the sun. Moreover, the motions of a straw in a eddy are not perfectly circular, and neither are those of the planets.
Illustration showing the vortex of the sun S surrounded by vortices of other stars.
The methodology of science elaborated by Descartes makes a number of assumptions: that we have knowledge of the simple of objects of mathematics and the complex objects constructed from them; that we know that God exists and what God is; that the physical world consists of quantifiable extended substances. Can these assumptions be justified? Descartes is best known for his attempt to place them (and others) on a solid footing, so that their use is not subject to any doubt.
Descartes's Meditations attempts to delineate what it is that we know and how it is that we know what we do. The attempt to state the extent of human knowledge runs up against an immediate problem: the human tendency to believe what is false. Before we can state what we know, we require a method for separating true knowledge from false belief.
Descartes compared the task of separating knowledge from false belief with that of separating the good apples from the bad apples in a basket. "Supposing he had a basket of apples and fearing that some of them were rotten, wanted to take those out lest they might make the rest go wrong, how could he do that? Would he not first turn the whole of the apples out of the basket and look them over one by one, and then having selected those which he saw not be rotten, place them again in the basket and leave out the others?" (Reply to seventh set of objections to the Meditations).
Corresponding to the dumping of the apples from the basket is a provisional rejection of those beliefs about which one has the slightest doubt. We treat them as false (we remove them from the basket) though we might later decide after all that some of them are true (we put the good apples back in). Descartes was able to cast doubt on nearly all his beliefs by producing a series of descriptions of how those beliefs might be false.
One must be careful in pursuing such a project. It is easy enough to dump a basket of apples on the ground and inspect each for freshness. The person doing the examination is not affected by the dumping of the apples. But if one were to reject all of one's beliefs, then there would be no basis for recovering any of them. For the beliefs are those of the person doing the examination, and emptying one's self of all beliefs would leave a person unable to justify anything.
The first stage of the method of doubt is easy and obvious. Like the ancient Greek philosophers, he noted that the senses are frequently deceptive: so much so that no belief based on mere sense-perception is free from doubt. What I think I see may be an illusion, a hallucination, a mistake made in haste, etc.
A classic response is to reserve our trust for only those perceptual beliefs which are subject to rational scrutiny. We can compensate for the unreliability of the senses by testing them by reason. The sun appears to be a small disk, but reason tells us that it is a huge sphere, etc. So while the unaided senses are unreliable, there is no reason to doubt the rational judgments we make on the basis of sense perception.
To this, Descartes replied that it is slightly doubtful whether there are any sensible objects at all. Because we cannot always distinguish perception of real sensible objects from dreaming, it is always possible in a given case that we are mistaken as to the existence of a thing outside our minds. And if it is possible in every individual case, it is possible that we are deceived in all cases. Thus he holds it as false (provisionally) that anything physical exists, including his own body.
Even though provisionally treating as false the existence of all physical bodies, Descartes retains beliefs about the objects of mathematics. Even if all is a dream, the images in the dream conform to something more general. They have shape, size, motion, which as we saw earlier are the objects of geometry. Perhaps our beliefs about the objects of arithmetic and geometry are not subject to doubt. If they are, then one of the pillars of scientific method would be disastrously removed.
Descartes makes them doubtful by noting the possibility of a being powerful enough to be able to deceive us about even the most obvious propositions of mathematics. God has the power to do this but would not because of the goodness intrinsic to God's nature. However, an evil demon might be powerful enough to do so. Given that this kind of being is possible, our mathematical judgments are uncertain, and we must declare them to be false, at least provisionally. But now we have called into question some of our most trustworthy faculties. Will there be any way out of the sceptical pit?
Since doubt has been cast on Descartes's beliefs about everything apart from himself, there remains only himself as something about which he could not be deceived. Here he finds the single point which remains fixed, and from which he might be able to rebuild his knowledge of other things. Without knowledge of himself, Descartes would be forced to conclude that the only indubitable belief is that nothing is certain: he would have to be a skeptic.
The impossibility of doubting his own existence is shown in the following argument. If he doubts that he has a body, he is doubting something. If he doubts whether '2 + 3 = 5' is true, he doubts something. And if he is doubting something, he must be an existing self which is engaged in doubting. Thus he asserts that it is necessary that 'I am' is true whenever he is thinking. But note that the possibility of a powerful demon may cast doubt on the principles which allow him to assert his own existence as a result of thinking. This is a critical problem for Descartes's theory of knowledge.
It should also be noted that the claim that one cannot doubt one's own existence is not original to Descartes. More than a thousand years earlier, Augustine had use a similar argument against contemporary skeptics. "If I am mistaken, then I exist. Certainly one who does not exist cannot be mistaken; consequently if I am mistaken, I exist."
This basis is found in the very activity of doubting itself. Thus Descartes ascribes to himself the properties of a being which has carried out the method. He doubts, understands, affirms, denies and denies. (We will call these cognitive functions of the mind.) In the process of doubting, he also exerts his will, as for example when he strives to avoid falling back into his own beliefs. (This is a volitional function of the mind.) The conclusion is that he is a thinking thing (Latin: res cogitans). His essence is thought, and since thought does not require a body, his body is not essential to him.
Not intrinsic to the process of doubting, but nonetheless not subject to doubt are mental images, which one directly experiences. Even if our experience is only a dream, Descartes cannot deny that he has mental images which are apparently the result of objects outside him. And he is able to imagine various things while in the process of doubting. Thus Descartes asserts that he is a being which imagines (produces images) and senses (apparently receives images). Note that sensing does not require the existence of physical senses, since the existence of his own body is still provisionally rejected.
Aristotle had given the senses an essential role in the knowledge of the physical world. Perception is a prerequisite to induction, whereby the form or universal is understood. But he was opposing a strong trend in ancient philosophy culminating in Plato, which held that there is nothing in the objects of the senses that is stable enough for knowledge.
Descartes is closer to the Platonic camp than to the Aristotelian. He attempted to show, through use of an example about the changes in a piece of wax, that the nature of the wax is known better through reason than though the senses. Over a period of time, a freshly produced piece of wax placed by the fire loses or changes all its specific properties, yet it is known to be the same object. Its taste and odor disappear, its color, size and shape are completely transformed, it loses its hardness and coldness to liquidity and warmth, etc.
The present perceptible state of an object is just one of many an object would have under different conditions. Thus to know the wax, one must be able to anticipate its changes. Here Descartes argues that the imagination (the faculty that produces images) could not generate a list of responses to all possible conditions, for there are infinitely many such conditions. Only the understanding is capable of grasping the general properties (extension, flexibility, mutability) responsible for changes in objects. So one knows the object best through the understanding rather through either faculty of images, sensation or imagination.
Descartes went on to note that any way of knowing an object (throught sensation, imagination, understanding) involves an act of the mind. And he claimed further that we can know these acts of mind better than we can know the objects of these acts. A consequence he drew was that the mind is better known than bodies. "There are no considerations that can aid in the perception of the wax or any other body without these considerations demonstrating even better the nature of my mind." Further, there are many ways, not involving bodies, by which we know our own minds best, as was shown at the beginning of the Second Meditation.
After having made his claims about his knowledge of himself, Descartes returned to the main thread of the Meditations. He had established that he could not doubt, when he was thinking, that he existed. Furthermore, he was able to catalogue the various activities which together comprise thinking. With respect to these things he was certain, not in the subjective sense of being sure or being very confident that he was right, but objectively. He could not be mistaken regarding these things that he knew about himself.
The only characteristic of this certainty, the mark by which it showed itself as certain, was the clarity and distinctness of the perception. In his Principles, Descartes had given these definitions: to be clear is just to be "present and manifest" to the mind, and to be distinct is to be distinguishable from everything else, and to have components that are all clear. In ordinary cases of vision, one clearly sees a cow ten feet in front of one's eyes, but not a rabbit in a bush or is a distant object. But in the context of the Third Meditation, Descartes is not concerned with the clarity of sense-perception, but rather of thoughts that occur in his understanding. That 2 + 3 = 5 is very clear and very distinct to the attentive mind.
Clarity and distinctness are supposed to be objective properties of thoughts that we have. Just as it is an objective fact whether one sees a cow clearly and distinctly with one's eyes, it is an objective fact whether one's clearly and distinctly comprehends that 2 + 3 = 5. At times, Descartes resorted to a metaphor of mental vision: a thought is an object before the mind's eye, and if the conditions are right, it will be "seen" just as it is. Descartes held that this clarity and distinctness are marks of certainty, and because certainty implies truth, he took clarity and distinctness to be the marks of truth.
But a complication arises when Descartes places his own existence on a part with 2 + 3 = 5 with respect to clarity and distinctness. "Yet every time I turn my attention to those very things that I think I perceive with such great clarity, I am so entirely persuaded by these things that I spontaneously burst out with these words: 'let him who can deceive me; as long as I think that I am something, he will never bring it about that I am nothing, . . . nor will he ever bring it about that two plus three yield more or less than five, or that similar matters, in which I recognize an obvious contradiction, exist." Despite the tenacity with which one holds onto these beliefs, the First Meditation has shown that there is a "very tenuous and, so to speak, metaphysical" reason for doubting even that 2 + 3 = 5. But then, how can clarity and distinctness themselves be a criterion of truth?
Perhaps the answer can be found in the way that Descartes tries to dispel the tenuous doubt about the simple propositions of mathematics. He tries to prove that he is not being deceived about what he clearly and distinctly perceives, on the grounds that God exists and would not permit such deception.
But if this is his strategy, Descartes seems to be caught in a circle of justification. If there is a possibility of doubt about what is clearly and distinctly perceived, then that doubt must be dispelled by proving that God would not allow us to be desceived about those matters. But to prove that God exists, one must rely on premises which must be known to be true. But how can they be known to be true unless one invokes some critierion? And if clarity and distinctness are the criteria, there is a circle, since their use depends on the proof of the existence of God.
There are in the Meditations two attempts to prove the existence of God. In the Third Meditation, there is a causal argument, moving from the existence of something (an idea of God) to its cause (God). The Fifth Meditation argument is not causal, but attempts to derive God's existence from the very nature of God. This nature is to be "eternal, infinite, omniscient, omnipotent, and creator of all things other than himself." Here is the basic causal argument for the existence of such a being.
Establishing 1. is not as easy as it might look. One can casually speak of infinity, but how do we know what an infinite being is? If it is formed from the notion of a finite being, removing the limitation, then why must an infinite being be the cause of such an idea? In fact, Descartes made the surprising claim that we know the infinite first and derive our idea of the finite from it.
Premise 2 is the key to the argument, and it requires a good deal of explanation. Suppose we grant that everything that exists has a cause (because such a principle is very clear and distinct, perceived by the "light of nature"). Then the idea of God has a cause, and the question is, which cause? Since I as a mind am capable of producing ideas through the use of the imagination, perhaps I myself am the cause of my idea of God! On the other hand, I may get this idea from other things, or it may be innate (inborn) in me.
Descartes argues that neither he, nor anything else other than God, can be the cause of this idea. This requires a principle concerning which ideas can be caused by which things. To differentiate ideas, Descartes appeals to their content, or "objective reality." If I have an idea of a giraffe, its content (a giraffe) is a finite creature, a mammal with a very long neck, hooved feet, etc. The cause of this idea, Descartes held, must be some actually existing thing, or "formal reality." In many cases, the formal reality of an object is the cause of the idea of that object, and is represented by its content. I am the formal reality which is the cause of the idea of myself, for example.
In the case of our idea of God, the objective reality of the idea reveals the formal reality of its cause: it is placed in our minds at creation, and hence is innate. But what aspect of the content of an idea could reveal its source? We have already seen that the ideas we have of physical objects, say my own body, may come from the body itself, or may come from my own imagination, as in a dream.
A beginning of an answer is to be found in a ranking of contents of ideas, which is paralleled by a ranking of formal realities by the same criteria. At the bottom we have modes of substance, which are less real than the substances which have them. There is more reality in a billiard ball than in its motion at some time, more reality in myself than in any individual thought I might have. Modes are modifications of the attributes of substance. The shape or size of a body depends on its extension, for example, and extension depends on substance. And there is a further hierarchy of substances. An infinite substance is more real than a finite substance.
So the content of my idea of God has the highest rank of objective reality. The key move in Descartes' argument is to claim that level of formal reality of the cause of an idea must be at least as great as the level of objective reality in the idea itself. I as a finite substance can cause the existence of an idea whose content is a mode, or even whose content is another finite substance. But I as a finite substance cannot cause the existence of an idea of an infinite substance. My formal reality is not great enough to produce an idea of such exalted content. There is only being, God, whose formal reality is great enough to produce an idea whose objective reality is God. So God exists.
Nonetheless, it might be thought that the fact that we commit so many errors shows that we are defective, so that God is responsible for our errorneous ways, and he hence engaged in a kind of deception. Descartes counters this possibility by claiming that we alone are responsible for our own errors: our faculties are such that if we use them as we should, we will never go wrong.
Error is the result of the application of unlimited will to the material given to us by a limited understanding. We can affirm, deny, or withhold judgment from whatever proposition we want (with the possible exception of those concerning our own existence and nature). But most of what we judge affirmatively is not sufficiently clear and distinct that it ought to be affirmed.
The question remains whether we really are free to judge in whatever way we please. Descartes thought that we can just see that to will is to act freely: the light of nature shows it to be the case that no external cause could force us to will as we do. But this claim is difficult, if not impossible to uphold. There always seems to be the possibility of some hidden cause for those of our actions which seem to us to lack them, as can be seen from cases of brainwashing or conditioning.
Descartes also thought that a free action can be undertaken without a reason sufficient to bring it about. We may be entirely indifferent with respect to two alternative courses, yet we may choose one of the two. This is the so-called "liberty of indifference," which Descartes thought to be the lowest form of freedom. Acting from the motive of choosing the best course is a higher form of liberty. There has been much resistance to this notion of indifference, going back at least to the medieval philosopher John Buridan, who said that an ass, presented with a bin of oats and one of barley, with no preference between the two, would starve to death. Spinoza was later to argue against the use of the possibility of indifference as a defense of free will.
St. Thomas Aquinas disagreed with the last step of Anselm's argument. Even if the concept of a non-existing God is incoherent, the only thing this tells us is something about our concepts. We cannot think a non-existing God, but this does not imply that there could not be one. "Now, from the fact that that which is indicated by the name God is conceived by the mind, it does not follow that God exists save only in the intellect. Hence that than whch a greater cannot be thought will likewise not have to exist save only in the intellect. From this it does not follow that there exists in reality something than which a greater cannot be thought" (Summa Contra Gentiles, Chapter 11). Aquinas himself advanced a number of causal arguments that God exists, though not the one used by Descartes in the Third Meditation. Descartes recognized that the Fifth Meditation argument from God's nature is the more difficult (Replies to Objections I).
Descartes' argument was not phrased in terms of what can be conceived, but rather in terms of our clear and distinct perception of the nature of God, which is something independent of our thinking. Thus he claimed that we have a clear and distinct perception the nature of a perfect being. We identify this nature with the nature of God. Further, existence is implied by such a nature, in that non-existence would be an imperfection. So God exists. But one must ask whether Descartes' claim that he clearly and distinctly perceives a "true and immutable nature" is justified. Why is this no more than saying that he has a concept of God as a perfect being?
The claim that we can perceive the very nature of God is a way of heading off Aquinas' objection that the greatest thinkable being may exist only in the intellect. Descartes could grant that if the conception of the greatest thinkable being exists only in the intellect, then any conclusion we draw from it is confined to the intellect as well. But the nature we perceive when we think of God is not an intellectual object; it has a reality of its own. "Necessary existence is comprised in the idea of a being of the highest power, not by any intellectual fiction, but because it belongs to the true and immutable nature of that being to exist. . . . That all-powerful being must comprise in himself all the other perfections that are contained in the idea of God, and hence these by their own nature and without any mental fiction are conjoined together and exist in God" (Replies to Objections I).
But how do we know that a nature corresponds to our idea of a perfect being? One way to answer this question is by showing that this idea is unique, that it is quite different from any idea we can fabricate on our own. For example, we can conceive of no other thing whose nature involves its existence. We can only conceive of one existing thing with such a nature. The existence of two supremely perfect beings is impossible, since they would limit each other and hence not be supremely perfect. There is nothing in the idea that could be added or subtracted. One still must ask why such an idea, unique though it may be, cannot be fabricated.
There is another famous objection to the ontological argument, this one given by Immanuel Kant in the eighteenth century. Descartes had held that to exist is a perfection, part of the nature of a perfect being. Kant claimed that existence is not part of the nature of anything, but can be attributed to something only in relation to experience. Suppose one were to grant that to exist would add something to a nature. Then there is something in the existing thing that is missing from the nature, in which case that nature is not what this existing thing really is. The nature and the existing thing must correspond exactly, so one could not be more perfect than another. An existing $100 has no properties that the nature of $100 dollars lacks, or it would not be an existing $100 but something else.
Having established to his satisfaction the existence of God, Descartes moved on to try to prove that extended things exist. Thus far, he has said much about what these things would be if they existed, but the doubts raised in the First Meditation about the unreliablity of the senses, the possibility of dreaming, and the possibility of radical deception have yet to be dispelled. With the existence of God in hand, Descartes thinks he is in a position to meet the doubts head-on.
But there seems to be no candidate for such a faculty. The mind is an entirely separate substance from the body. Descartes claims that he can think of them apart from each other, so they can exist apart. Whatever can exist apart God creates so that they do so exist. Imagining and "sensing" are not of his essence but only depend on thinking. We cannot explain the presence of images through anything essential to the mind, so the best explanation of them is that they are caused by something external, either bodies whose formal reality (see Meditation 4) corresponds to the objective reality of the ideas, that is, extended things, or else by God or some other higher being.
But if a higher spiritual being is the cause (as George Berkeley was to claim in the 18th century), then God would be a deceiver. Descartes reports a great inclination to believe that the cause of the ideas whose content is extension and its modes is an extended thing. If God had made him such that he were wrong about this, God would be deceiving him. But God is no deceiver. Thus our knowledge of the existence of extended things depends on our knowledge of the existence of God. This is a remarkable conclusion. David Hume wrote that "To have recourse to the veracity of the supreme Being, in order to prove the veracity of our senses, is surely making a very unexpected circuit" (Enquiry Concerning Human Understanding, Chapter XII).
Given that we do have bodies, the last question is how it is that the body conveys to the mind information through "sense." A central question is whether there is a single point at which all the information conveyed by the nerves converges, or whether the mind collects information throughout the body by being, as it were, mingled with it. An argument for the "mingling" view is that we do not observe changes in our bodies in a detached way, as a pilot might view the cracking of the mast on his ship, but rather are intimately connected with it.
On the other hand, the "single point" account of information collection has a certain advantage, because it can explain how it is that the mind frequently errs in its judgments about what is going on in one's own body and about what is going on in the world outside one's own body. If he mind's access to the body is limited, if there is a single point in the brain which collects all the information conveyed by the nerves, then there are various ways in which the same information can be initiated from different causes. The collection point can receive motions from one point which mimic other motions which arise from another point. For example, the motion caused in a painful foot can be caused by something between the foot and the brain, even if the body has no foot.
The fact that we do not have infallible information about anything going on outside our minds does not imply that God, our creator, is a deceiver. Our bodies convey information well enough for the purposes of life. However, there are cases where our natural inclinations to believe and act would do us great harm, as when a person with dropsy feels very thirsty, though drinking would be very harmful.
Still, we can correct our harmful impulses through the use of reason. This is the chief remedy against all the deception of the senses, and even the possibility that we are constantly dreaming. Descartes recognizes that there is a coherence in waking life that is lacking in any dream. Using our reason when we are awake, we can relate our present experience with many others we can remember. With this cirterion in hand, we are rid at last of the hypothesis that all is a dream.
Gassendi was an atomist who did not believe in the separate existence of mind and body. He disputed Descartes's assertion that he is both thinking and unextended, arguing that any description of the mind as being united to the body has the consequence that the mind is extended. If it is to the whole body, then how can the mind fail to have parts, since the mind is said to be in a number of parts of the body? The same objection holds for the attempt to locate the mind in any smaller part of the body, such as the brain or one of its parts. On the other hand, if the mind is confined to a mathematical point, then there is no way to account for interaction with physical components of the body, all of which are extended.
"For if you are not greater than a point, how can you be united with the entire body, which is of such great magnitude? How, at least, can you be united with the brain, or some minuete part of it, which (as has been said) must yet have some magnitude or extension, however small it be? If you are wholly without parts, how can you mix or appear to mix with its minute subdivisions? For there is no mixture unless each of the things to be mixed has part which can mix with one another?" etc.
Descartes did not say much in response, on the grounds that Gassendi produced not objections, but only doubts stemming from Gassendi's interpretation of Descartes. That is, "they arise merely from your wishing to subject to the scrutiny of the imagination matters, which, by their own nature, do not fall under it." To compare the mixture of the parts of bodies with the union of body and mind is wrong, because body and mind are "wholly diverse."
But given this hard line, what can we make of Descartes's descriptions of the "union" and "intermingling" of the mind with the body? And how can the interaction of mind and body be explained? Elisabeth framed the problem this way: If you exclude extension as a property of the mind, there can be no contact between body and soul, since contact presupposes extension. But bodily motion is induced by contact by extended bodies, so how can the will induce motion?
Descartes's answer to Elisabeth was nothing if not puzzling. After explaining how the notions of mind, body and their union must not be confused, he used an example of weight, claiming that the ordinary way of understanding it is an abuse of "what has been given us for conceiving the manner in which the soul moves the body." In response to this, Elisabeth discreetly asked to be excused for her "stupidity in being unable to comprehend, from what you had previously said concerning weight, the idea by which we should judge the soul (nonextended and immaterial) can move the body." In fact, Descartes was presupposing his discussion in the reply to the Sixth Objections, to which he referred in his letter.
There he had claimed that we use our conceptions of the intellectual to explain what belongs to the body: "the ideas of those things which I supposed to be corporeal were formed and conceived in such a way as to refer to minds rather than bodies." Weight is a case in point. We have noted that according to Aristotle, whom Descartes studied in his youth, earthy bodies move toward their natural place, the center of the earth. "I thought that gravity carried bodies toward the center of the earth as if it contained some knowledge of this center within it." A more subtle way in which he had conceived weight on the model of the intellect is in his belief that "gravity was diffused throughout the whole of the body possessing weight," yet "while it remained coextensive with the heavy body, it could exercise its force at any point of that body." A it does not matter where on a heavy ball a rope is attached: its pulling force is exactly the same.
The origin of these beliefs about weight is explained by the fact that weight is related to bodies just as minds are. "Indeed it is in no other way that I now understand mind to be coextensive with the body, the whole [mind] in the whole [body], and the whole [mind] in any of its parts." This suggests that if it is "coextensive" with bodies, the mind has a kind of extension that is distinct from the extension of bodies. The mind may be diffused through the body yet not be extended in the way a body is, "for true bodily extension is of such a nature to prevent any interpenetration of parts."
So when Elisabeth complained that it was easier for her to think of the mind as corporeal than to comprehend how an incorporeal thing could have any effect on a body, Descartes could respond, "I ask her to please freely attribute this matter and this extension to the soul; for it is nothing but to conceive it united to the body." The reason the attribution can be made is that "the extension of this matter is of another nature than the extension of this thought, in that the first is determined to a certain place, from which it excludes every other extension of body, which the second does not." Elisabeth was not convinced by this evidently. She proposed that the mind has an "unknown property" of extension. And it is not the special sort of extension proposed by Descartes, in which the whole mind is in the whole body as well as in all of its parts: Elisabeth rightly referred to that notion as a "scholastic contradiction."
A few more words about extension might be helpful. The generic notion of extension is that of having dimension. A mathematical point lacks length, breadh and depth, so it is unextended. What is extended has at least one dimension (as does a line), two dimensions, (as does a plane) or three dimensions (as does a solid). The extension of a body is three-dimensional. Moreover, the extension of a body must be distinguished from that of an incorporeal solid: corporeal extension excludes all else from its interior. (It is impenetrable.) An extended mind, then, would be three-dimensional but not impenetrable. But note that in the Passions of the Soul, written after the Elisabeth correspondence, Descartes also stated that the soul is "of such a nature which has no relation to extension, nor dimensions, nor other properties of matter of which the body is composed" (Article XXX). Nonetheless, "The soul is really joined to the whole body."
The concession Descartes made to Elisabeth seems to be based on his desire to describe an object, the "union" of mind and body, which is something in addition to the mind and body themselves. But there is nothing said about the way in which this object is supposed to function, other than that it acts "as if" the mind were concentrated at a single point. This, and the repudiation of the extension of the mind, lead us back to the way in which Descartes described the interaction of mind and body as being highly localized.
Descartes is best known for his view that there is one place in the brain, the pineal gland, which is the end-point of sensory input and the beginning-point of voluntary actions. Margaret Cavendish (not sympathetically) used an analogy with a spider to describe this setup. Corresponding to the strands of a spider's web are the ducts through which flow the "animal spirits." Any change in an outer part of the body is communicated to the command center, which sits delicately poised to be moved by the smallest disturbance and to move the animal spirits considerably by the slightest movement of its own. (Passions of the Soul, Article XXXI).
This physical description of the center of bodily movement is then augmented by the claim that the mind is capable of being affected by physical motion as well as affecting the motion. In the latter case, "The whole action of the soul consists in this, that solely because it desires something, it causes the little gland to which it is closely united to move in the way requisite to produce the effect which relates to this desire" (Passions of the Soul, Article XLI).
Leibniz criticized Descartes' contention that an act of will can cause motion in the animal spirits. Descartes himself had laid down a law of the conservation of motion . If the will injects new motions into the system of the world, as a kind of unmoved mover, then that quantity would be increased. The problem might be finessed if it is held that only the direction of motion, not the quantity of motion is changed by the body (a "martial arts") solution). But Leibniz showed that the product of direction and quantity is preserved. So, "Apart from the fact that the physical influence of one of these substances [mind and body] on the other is inexplicable, I recognized that without a complete derangement of the laws of Nature the soul could not act physically upon the body" (Theodicy, Part I, Section 61).
Marleen Rozemond has argued that traditional scholarship has misplaced Descartes' concerns about the relation between mind and body. The objections raised by Gassendi and Elisabeth both concern the "heterogeneity" of mind and body, i.e., the fact that their principal attributes (thinking and extension, respectively) are unlike each other. Descartes responded to these objections only grudgingly. His real interest, she maintains, is in the fact that ideas in the mind do not resemble their extended counterparts. This is particularly so with ideas such as color and heat, but it even holds of the geometrical properties of bodies such as size, shape and motion.
Because of the dissimilarity between ideas and the motions and shapes they represent, we must conclude that our ideas are innate. "Nothing arrived at our minds from extenal objects through the organs of sense except for certain corporal motions. . . . But not even those motions themselves and that shapes that arise form them are conceived by us as they come to be in the organs of the senses. . . . Hence it follows that the ideas themselves of motions and shapes are innate in us" (Comments on a Certain Broadsheet). It is only on the occasion of the motions that the ideas are produced by the mind. The mind is the "proximate" cause of these ideas, while the bodily motions are only "remote" causes. The motions are still essential to the process of producing ideas, because it is they which determine which innate ideas will be triggered in their presence.
The doctrine that bodily motions are only the occasions and not the primary causes of ideas was developed most fully by the Cartesian philosopher Nicolas Malebranche (1638-1715). His doctrine, known as "occasionalism," should not be confused with that suggested by Descartes here. Our ideas of extended things are innate and are triggered by the presence in the mind of sensation, which is in turn is caused by God, rather than bodies. "The sensation is a modification of the soul, and it is God who causes it in us. . . . As for the idea found in conjunction with the sensation, it is in God, and we see it because it pleases God to reveal it to us. God joins the sensation to the idea when objects are present so that we may believe them to be present and that we may have all the feelings and passions that we should have in relation to them" (The Search after Truth, Book Three, Part Two, Chapter Six).
Philosophy 1 Lecture notes on the Meditations
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