LIFE 2. Your Bodys Time Machine. (Alternative Methods of Nature Book 1)

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View or edit your browsing history. Get to Know Us. English Choose a language for shopping. Not Enabled Word Wise: Not Enabled Screen Reader: Enabled Amazon Best Sellers Rank: Would you like to report this content as inappropriate? As regards parsimony or simplicity, he pointed out that his reconceived matter had only a few basic properties especially size, shape, position, and motion , from which he would construct his explanations. He claimed great explanatory scope by contending that his explanations could extend to all natural phenomena, celestial and terrestrial, inorganic and organic.

But throughout the s, Descartes claimed that he also was in possession of a metaphysics that could justify the first principles of his physics, which he finally presented in the Meditations and Principles. Some scholars emphasize the epistemological aspects of Descartes' work, starting with the Rules and continuing through to the Principles. Accordingly, the main change in Descartes' intellectual development is the introduction of skeptical arguments in the Discourse and Meditations.

Many interpreters, represented prominently in the latter twentieth century by Richard Popkin , believe that Descartes took the skeptical threat to knowledge quite seriously and sought to overcome it in the Meditations.


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By contrast, in the main interpretive thread followed here, skeptical arguments were a cognitive tool that Descartes used in order to guide the reader of the Meditations into the right cognitive frame of mind for grasping the first truths of metaphysics. Achieving stable knowledge of such truths would have as a side-effect security against skeptical challenge. The reader who is curious about these issues should read the relevant works of Descartes, together with his correspondence from the latter half of the s and early s.

René Descartes

Descartes first presented his metaphysics in the Meditations and then reformulated it in textbook-format in the Principles. His metaphysics sought to answer these philosophical questions: How does the human mind acquire knowledge? What is the mark of truth? What is the actual nature of reality? How are our experiences related to our bodies and brains? Is there a benevolent God, and if so, how can we reconcile his existence with the facts of illness, error, and immoral actions? Descartes had no doubt that human beings know some things and are capable of discovering others, including at least since his metaphysical insights of fundamental truths about the basic structure of reality.

Yet he also believed that the philosophical methods taught in the schools of his time and used by most of his contemporaries were deeply flawed. He believed that the doctrines of scholastic Aristotelian philosophy contained a basic error about the manner in which fundamental truths, such as the truths of metaphysics, are to be gained. He expressed this mistaken view in the First Meditation, by saying not in his own voice, but in a voice for the reader: He then went on to challenge the veridicality of the senses with the skeptical arguments of First Meditation, including arguments from previous errors, the dream argument, and the argument from a deceptive God or an evil deceiver.

Descartes explained these convictions as the results of childhood prejudice 7: As children, we are naturally led by our senses in seeking benefits and avoiding bodily harms. Descartes denied that the senses reveal the natures of substances. He held that in fact the human intellect is able to perceive the nature of reality through a purely intellectual perception. Descartes constructed the Meditations so as to secure this process of withdrawal from the senses in Meditation I.

Meditation II brings the discovery of an initial truth, in the cogito 7: Hence, he sets up clear and distinct intellectual perception, independent of the senses, as the mark of truth 7: We consider these results in Secs. For now, let us examine what Descartes thought about the senses as a source of knowledge that was different from the pure intellect. In the Meditations , he held that the essence of matter could be apprehended by innate ideas, independently of any sensory image 7: To that extent, his later position agrees with the Platonic tradition in philosophy, which denigrated sensory knowledge and held that the things known by the intellect have a higher reality than the objects of the senses.

Descartes, however, was no Platonist, a point to which we will return. His attitude toward the senses in his mature period was not one of total disparagement. Descartes assigned two roles to the senses in the acquisition of human knowledge. First, he acknowledged that the senses are usually adequate for detecting benefits and harms for the body. In this connection, he was agreeing with the conception of the function of the senses that was widely shared in the traditional literature in natural philosophy, including the Aristotelian literature, as well as in the medical literature on the natural functions of the senses.

Second, he recognized that the senses have an essential role to play in natural philosophy. The older interpretive literature sometimes had Descartes claiming that he could derive all natural philosophical or scientific knowledge from the pure intellect, independent of the senses.

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But Descartes knew full well that he could not do that. He distinguished between the general principles of his physics and the more particular mechanisms that he posited to explain natural phenomena, such as magnetism or the properties of oil and water. These include the fundamental doctrine that the essence of matter is extension Princ. As to particular phenomena, in general he had to rely on observations to determine their properties such as the properties of the magnet , and he acknowledged that multiple hypotheses about subvisible mechanisms could be constructed to account for those phenomena.

The natural philosopher must, therefore, test the various hypotheses by their consequences, and consider empirical virtues such as simplicity and scope Disc. Further, Descartes knew that some problems rely on measurements that can only be made with the senses, including determining the size of the sun 7: Although Descartes recognized an important role for the senses in natural philosophy, he also limited the role of sense-based knowledge by comparison with Aristotelian epistemology.

According to many scholastic Aristotelians, all intellectual content arises through a process of intellectual abstraction that starts from sensory images as present in the faculty of imagination. Mathematical objects are formed by abstraction from such images. Even metaphysics rests on knowledge derived by abstraction from images. Of course, in this Aristotelian scheme the intellect plays an important role in grasping mathematical objects or the essences of natural things through considering images. By contrast, Descartes affirmed that the truths of mathematics and metaphysics are grasped by the intellect operating independently of the senses and without need for assistance from the faculty of imagination.

In Descartes' scheme of mental capacities, knowledge does not arise from the intellect alone. The intellect may present some content as true, but by itself it does not affirm or deny that truth. That function belongs to the will.

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A judgment, and hence an instance of at least putative knowledge, does not arise in this scheme until the will has affirmed or denied the content presented by the intellect. The intellect is the power of perception or representation. Acts of pure intellect occur without the need for any accompanying brain processes; these are purely intellectual perceptions. But there are other intellectual acts that require the presence of the body: These intellectual acts are less clear and distinct than acts of pure intellect, and may indeed be obscure and confused as in the case of color sensations.

Nonetheless, the will may affirm or deny such content. As discussed in the next subsection, error can arise in these judgments. In sum, in considering Descartes' answer to how we know, we can distinguish classes of knowledge that differ as regards the degree of certainty one may expect to achieve. Metaphysical first principles as known by the intellect acting alone should attain absolute certainty. Practical knowledge concerning immediate benefits and harms is known by the senses. Such knowledge is usually good enough. Objects of natural science are known by a combination of pure intellect and sensory observation: For submicroscopic particles, we must reason from observed effects to potential cause.


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In these latter cases, our measurements and our inferences may be subject to error, but we may also hope to arrive at the truth. Clarity and distinctness of intellectual perception is the mark of truth. In the fifth set of Objections to the Meditations , Gassendi suggests that there is difficulty concerning. Gassendi has in effect asked how it is that we should recognize clear and distinct perceptions. If clarity and distinctness is the mark of truth, what is the method for recognizing clarity and distinctness? In reply, Descartes claims that he has already supplied such a method 7: What could he have in mind?

It cannot be the simple belief that one has attained clarity and distinctness, for Descartes himself acknowledges that individuals can be wrong in that belief 7: Nonetheless, he does offer a criterion. We have a clear and distinct perception of something if, when we consider it, we cannot doubt it 7: That is, in the face of genuine clear and distinct perception, our affirmation of it is so firm that it cannot be shaken, even by a concerted effort to call the things thus affirmed into doubt.

As mentioned in 3. The intellect perceives or represents the content of the judgment; the will affirms or denies that content. The inclination of the will is so strong that it amounts to compulsion; we cannot help but so affirm. Descartes thus makes unshakable conviction the criterion. Can't someone be unshakable in their conviction merely because they are stubborn? But Descartes is talking about a conviction that remains unshakable in face of serious and well-thought out challenges 7: To be immune from doubt does not mean simply that you do not doubt a proposition, or even that it resists a momentary attempt to doubt; the real criterion for truth is that the content of a proposition is so clearly perceived that the will is drawn to it in such a way that the will's affirmation cannot be shaken even by the systematic and sustained doubts of the Meditations.

Perhaps because the process for achieving knowledge of fundamental truths requires sustained, systematic doubt, Descartes indicates that such doubt should be undertaken only once in the course of a life 7: Even so, problems remain. Having extracted clarity and distinctness as the criterion of truth at the beginning of the Third Meditation, Descartes immediately calls it into question. He re-introduces an element of the radical doubt from the First Meditation: In the course of the Third Meditation, Descartes constructs an argument for the existence of God that starts from the fact that he has an idea of an infinite being.

The argument is intricate. Descartes then applies that principle not to the mere existence of the idea of God as a state of mind, but to the content of that idea. Descartes characterizes that content as infinite, and he then argues that a content that represents infinity requires an infinite being as its cause. He concludes, therefore, that an infinite being, or God, must exist. He then equates an infinite being with a perfect being and asks whether a perfect being could be a deceiver.

The second and fourth sets of objections drew attention to a problematic characteristic of this argument. In the words of Arnauld:. Arnauld here raises the well-known problem of the Cartesian circle, which has been much discussed by commentators in recent years. In reply to Arnauld, Descartes claims that he avoided this problem by distinguishing between present clear and distinct perceptions and those that are merely remembered 7: He is not here challenging the reliability of memory Frankfurt Rather, his strategy is to suggest that the hypothesis of a deceiving God can only present itself when we are not clearly and distinctly perceiving the infinity and perfection of God, because when we are doing that we cannot help but believe that God is no deceiver.

It is as if this very evident perception is then to be balanced with the uncertain opinion that God might be a deceiver 7: The evident perception wins out and the doubt is removed. Descartes explicitly responds to the charge of circularity in the manner just described. Over the years, scholars have debated whether this response is adequate. Some scholars have constructed other responses on Descartes' behalf or have found such responses embedded in his text at various locations.

One type of response appeals to a distinction between the natural light and clear and distinct perception, and seeks to vindicate the natural light without appeal to God Jacquette Another response suggests that, in the end, Descartes was not aiming at metaphysical certainty concerning a mind-independent world but was merely seeking an internally coherent set of beliefs Frankfurt A related response suggests that Descartes was after mere psychological certainty Loeb The interested reader can follow up this question by turning to the literature here cited as also Carriero , Doney , and Hatfield Building on his claim that clear and distinct perceptions are true, Descartes seeks to establish various results concerning the nature of reality, including the existence of a perfect God as well as the natures of mind and matter to which we turn in the next subsection.

Here we must ask: What is the human mind that it can perceive the nature of reality? Descartes has a specific answer to this question: For readers in Descartes' day, this claim would naturally raise a further question: Descartes rejected both alternatives.

He denied, along with many of his contemporaries, that there are eternal truths independent of the existence of God. But he also denied that the eternal truths are fixed in God's intellect. Some Neoplatonist philosophers held that the eternal truths in the human mind are copies, or ectypes, of the archetypes in the mind of God. Eternal truths are latent in God's creative power, and he understands this, so that if human beings understand the eternal truths as eternal, they also do so by understanding the creative power of God Hatfield Descartes had a different account.

He held that the eternal truths are the free creations of God 1: He might have created other essences, although we are unable to conceive what they might have been. Our conceptual capacity is limited to the innate ideas that God has implanted in us, and these reflect the actual truths that he created. God creates the eternal truths concerning logic, mathematics, the nature of the good, the essences of mind and matter , and he creates the human mind and provisions it with innate ideas that correspond to those truths.

However, even in this scheme there must remain some eternal truths that are not created by God: Descartes reveals his ontology implicitly in the Meditations , more formally in the Replies, and in textbook fashion in the Principles. The main metaphysical results that describe the nature of reality assert the existence of three substances, each characterized by an essence. The first and primary substance is God, whose essence is perfection. In fact, God is the only true substance, that is, the only being that is capable of existing on its own. Descartes' arguments to establish the essences of these substances appeal directly to his clear and distinct perception of those essences.

The essence of matter is extension in length, breadth, and depth. Cartesian matter does not fill a distinct spatial container; rather, spatial extension is constituted by extended matter there is no void, or unfilled space. Modes are properties that exist only as modifications of the essential principal and the general attributes of a substance.

In addition to its essence, extension, matter also has the general attributes of existence and duration. The individual parts of matter have durations as particular modes. All the modes of matter, including size, shape, position, and motion, can exist only as modifications of extended substance. The essence of mind is thought. Besides existence and duration, minds have the two chief powers or faculties previously mentioned: The intellectual or perceiving power is further divided into the modes of pure intellect, imagination, and sense perception.

Pure intellect operates independently of the brain or body; imagination and sense perception depend upon the body for their operation as does corporeal memory. The will is also divided into various modes, including desire, aversion, assertion, denial, and doubt. These always require some intellectual content whether pure, imagined, or sensory upon which to operate. It seems he held that the mind essentially has a will, but that the intellectual or perceptive, or representational power is more basic, because the will depends on it in its operation.

What role does consciousness play in Descartes' theory of mind? Many scholars believe that, for Descartes, consciousness is the defining property of mind e. There is some support for this position in the Second Replies. If mind is thinking substance and thoughts are essentially conscious, perhaps consciousness is the essence of thought? Descartes in fact did hold that all thoughts are, in some way, conscious 7: He did not mean by this that we have reflective awareness of, and can remember, every thought that we have 5: In the Second Meditation, he describes himself as a thinking thing by enumerating all the modes of thoughts of which he is conscious: He thus sets up consciousness as a mark of thought.

But is it the essence? There is another possibility. If perception intellection, representation is the essence of thought, then all thoughts might be conscious in a basic way because the character of the intellectual substance is to represent, and any representation present in an intellectual substance is thereby conscious. Similarly, any act of will present in an intellectual substance also is available to consciousness, because it is of the essence of such a substance to perceive its own states Accordingly, perception or representation is the essence of mind, and consciousness follows as a result of the mind's being a representing substance.

All the same, in distinguishing between thoughts possessed of consciousness and thoughts of which we are reflectively aware, Descartes opened a space for conscious thoughts that we don't notice or remember. As in his theory of the senses Sec. In the Discourse , Descartes presented the following argument to establish that mind and body are distinct substances:. This argument moves from the fact that he can doubt the existence of the material world, but cannot doubt the existence of himself as a thinking thing, to the conclusion that his thoughts belong to a nonspatial substance that is distinct from matter.

The argument is fallacious. It relies on conceivability based in ignorance. Descartes has not included anything in the argument to ward off the possibility that he, as a thinking thing, is in fact a complex material system. He has merely relied on the fact that he can doubt the existence of matter to conclude that matter is distinct from mind. This argument is clearly inconclusive. From the fact that the Joker cannot, at a certain moment, doubt the existence of Batman because he is with him , but he can doubt the existence of Bruce Wayne who might, for all the Joker knows, have been killed by the Joker's henchmen , it does not follow that Bruce Wayne is not Batman.

In fact, he is Batman. The Joker is merely ignorant of that fact. In the Meditations , Descartes changed the structure of the argument. In the Second Meditation, he established that he could not doubt the existence of himself as a thinking thing, but that he could doubt the existence of matter. However, he explicitly refused to use this situation to conclude that his mind was distinct from body, on the grounds that he was still ignorant of his nature 7: Then, in the Sixth Meditation, having established, to his satisfaction, the mark of truth, he used that mark to frame a positive argument to the effect that the essence of mind is thought and that a thinking thing is unextended; and that the essence of matter is extension and that extended things cannot think 7: He based this argument on clear and distinct intellectual perceptions of the essences of mind and matter, not on the fact that he could doubt the existence of one or the other.

This conclusion in the Sixth Meditation asserts the well-known substance dualism of Descartes. That dualism leads to problems. As Princess Elisabeth, among others, asked: This problem vexed not only Descartes, who admitted to Elisabeth that he didn't have a good answer 3: It seems that, somehow, states of the mind and the body must be brought into relation, because when we decide to pick up a pencil our arm actually moves, and when light hits our eyes we experience the visible world.

But how do mind and body interact? Some of Descartes' followers adopted an occasionalist position, according to which God mediates the causal relations between mind and body; mind does not affect body, and body does not affect mind, but God gives the mind appropriate sensations at the right moment, and he makes the body move by putting it into the correct brain states at a moment that corresponds to the volition to pick up the pencil. Other philosophers adopted yet other solutions, including the monism of Spinoza and the pre-established harmony of Leibniz. In the Meditations and Principles , Descartes did not focus on the metaphysical question of how mind and body interact.

Rather, he discussed the functional role of mind—body union in the economy of life. As it happens, our sensations serve us well in avoiding harms and pursuing benefits. Pain-sensations warn us of bodily damage. Pleasure leads us to approach things that usually are good for us. Our sense perceptions are reliable enough that we can distinguish objects that need distinguishing, and we can navigate as we move about. They are not perfect. Sometimes our senses present things differently than they are, and sometimes we make judgments about sensory things that extend beyond the appropriate use of the senses.

In discussing the mark of truth, Descartes suggested that the human intellect is generally reliable because it was created by God. In discussing the functioning of the senses to preserve or maintain the body, he explained that God has arranged the rules of mind—body interaction in such a manner as to produce sensations that generally are conducive to the good of the body.

Nonetheless, in each case, errors occur. In various circumstances, our judgments may be false often, about sensory things , just as, more broadly, human beings make poor moral choices, even though God has given them a will that is intrinsically drawn to the good 1: In addition, our sense perceptions may represent things as being a certain way, when they are not.

Sometimes we feel pain because a nerve has been damaged somewhere along its length, and yet there is no tissue damage at the place in which the pain is felt. Amputees may feel pain in their fingers when they have no fingers Princ. Descartes responded to these problems differently. He explained cognitive and moral errors as resulting from human freedom. God provides human beings with a will, and wills are intrinsically free. In this way, there is no difference in degree in freedom between God and man.

But human beings have finite intellects. And because they are free, they can choose to judge in cognitive or moral situations for which they do not have clear and distinct perceptions of the true or the good. If human beings restricted their acts of will to cases of clear and distinct perception, they would never err. But the vicissitudes of life may require judgments in less than optimal circumstances, or we may decide to judge even though we lack a clear perception.

In either case, we may go wrong. Matters are different for the errors of sensory representation. The senses depend on media and sense organs and on nerves that must run from the exterior of the body into the brain. God sets up the mind—body relation so that our sensations are good guides for most circumstances. But the media may be poor the light may not be good , circumstances may be unusual as with the partially submerged stick that appears as if bent , or the nerves may be damaged as with the amputee.

In these cases, the reports of the senses are suboptimal. Since God has set up the system of mind—body union, shouldn't God be held accountable for the fact that the senses can misrepresent how things are? Here Descartes does not appeal to our freedom not to attend to the senses, for in fact we must often use the senses in suboptimal cognitive circumstances when navigating through life. Rather, he points out that God was working with the finite mechanisms of the human body 7: In fact, the distinction between these two types of error, cognitive error and sensory misrepresentation, is not completely clear-cut in Descartes.

In the case of the amputee, the pain seems to be in fingers that are not there. That appears to be a clear case of sensory misrepresentation: Similarly with the partially submerged stick. It may look bent. In these cases, even if we use our intellects to interpret the illusions or sensory misrepresentations so as to avoid error by withholding judgment or even by judging correctly 7: In other cases, however, Descartes describes the senses as providing material for error, but it remains uncertain whether he assimilates such error to what has been labelled cognitive error or to sensory misrepresentation.

He offers as an example the idea of cold: Accordingly, this case should be assimilated to sensory misrepresentation: Material falsity would be a matter of misrepresentation. But Descartes also offers a different gloss on the obscurity of sensory ideas. Accordingly, sensory ideas are not misrepresentations, they are simply so obscure and confused that we cannot tell what their representational content might be by considering their experienced character, such as the phenomenal character of cold or of color.

Metaphysics and natural philosophy are needed to tell us what our color sensations obscurely represent: On this interpretation, Descartes is saying that the resemblance thesis arises not because the sensory ideas of cold or of color misrepresent those qualities in objects, but because we make a cognitive error, stemming from the prejudices of childhood as mentioned in Sec.

The issues surrounding the notion of material falsity in Descartes are intricate and cut to the core of his theory of mind and of sensory representation. The interested reader can gain entrance to literature through Wee and Hatfield More generally, Copernicus had, in the previous century, offered a forceful argument for believing that the sun, not the earth, is at the center of the solar system.

Early in the seventeenth century, Johannes Kepler announced new results in optics, concerning the formation of images, the theory of lenses, and the fact that the retinal image plays a central role in vision. By the early s, Descartes was aware 1: Descartes himself contributed some specific new results to the mathematical description of nature, as co-discoverer of the sine law of refraction and as developer of an accurate model of the rainbow. Special physics concerned actually existing natural entities, divided into inanimate and animate.

Inanimate physics further divided into celestial and terrestrial, in accordance with the Aristotelian belief that the earth was at the center of the universe, and that the earth was of a different nature than the heavens including the moon, and everything beyond it. Animate terrestrial physics concerned the various powers that Aristotelians ascribed to ensouled beings, where the soul is considered as a principle of life possessing vital as well as mental or cognitive powers.

In the simplest textbooks, the powers of the soul were divided into three groups: Descartes' ambition was to provide replacements for all the main parts of Aristotelian physics. In his physics, there is only one matter and it has no active forms. Thus, he dissolved the boundary that had made the celestial and the terrestrial differ in kind. His one matter had only the properties of size, shape, position, and motion. The matter is infinitely divisible and it constitutes space; there is no void, hence no spatial container distinct from matter.

The motions of matter are governed by three laws of motion, including a precursor to Newton's law of inertia but without the notion of vector forces and a law of impact. Earth, air, fire, and water were simply four among many natural kinds, all distinguished simply by the characteristic sizes, shapes, positions, and motions of their parts. Although Descartes nominally subscribed to the biblical story of creation, in his natural philosophy he presented the hypothesis that the universe began as a chaotic soup of particles in motion and that everything else was subsequently formed as a result of patterns that developed within this moving matter.

Thus, he conceived that many suns formed, around which planets coalesced. On these planets, mountains and seas formed, as did metals, magnets, and atmospheric phenomena such as clouds and rain. The planets themselves are carried around the sun in their orbits by a fluid medium that rotates like a whirlpool or vortex.

Rather, they are driven down by the whirling particles of the surrounding ether. Descartes insisted that all cases of apparent action at a distance, including magnetism, must be explained through the contact of particle on particle. He explained magnetism as the result of corkscrew-shaped particles that spew forth from the poles of the earth and flow from north to south or vice versa, causing magnetized needles to align with their flow Princ. To explain magnetic polarity, Descartes posited that the particles exiting from the south pole are threaded in one direction and those from the north are threaded oppositely like the oppositely threaded spindles on bicycle pedals.

Descartes also wanted to provide an account of the formation of plants and animals by mechanical causes, but he did not succeed during his lifetime in framing an account that he was willing to publish so that only portions of his physiology were revealed in the Discourse , Dioptrics , Meditations , Principles , and Passions. In writings that were published only posthumously but were read by friends and followers during his lifetime, e. In mechanizing the concept of living thing, Descartes did not deny the distinction between living and nonliving, but he did redraw the line between ensouled and unensouled beings.

In his view, among earthly beings only humans have souls. He thus equated soul with mind: Descartes regarded nonhuman animals as machines, devoid of mind and consciousness, and hence lacking in sentience. Although Descartes' followers understood him to have denied all feeling to animals, some recent scholars question this interpretation; on this controversy, see Cottingham and Hatfield Consequently, Descartes was required to explain all of the powers that Aristotelians had ascribed to the vegetative and sensitive soul by means of purely material and mechanistic processes These mechanistic explanations extended, then, not merely to nutrition, growth, and reproduction, but also to the functions of the external and internal senses, including the ability of nonhuman animals to respond via their sense organs in a situationally appropriate manner: In the Treatise on Man and Passions , Descartes described purely mechanical processes in the sense organs, brain, and muscles, that were to account for the functions of the sensitive soul.

The brain structures that mediate behavior may be innate or acquired. Descartes ascribed some things that animals do to instinct; other aspects of their behavior he explained through a kind of mechanistic associative memory. He held that human physiology is similar to nonhuman animal physiology, as regards both vegetative and some sensitive functions—those sensitive functions that do not involve consciousness or intelligence:.


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Many of the behaviors of human beings are actually carried out without intervention from the mind. The fact that Descartes offered mechanistic explanations for many features of nature does not mean that his explanations were successful. Indeed, his followers and detractors debated the success of his various proposals for nearly a century after his death. His accounts of magnetism and gravity were challenged. Leibniz challenged the coherence of Descartes' laws of motion and impact.

Newton offered his own laws of motion and an inverse square law of gravitational attraction. His account of orbital planetary motions replaced Descartes' vortexes. Others struggled to make Descartes' physiology work. There were also deeper challenges. Some wondered whether Descartes could actually explain how his infinitely divisible matter could coalesce into solid bodies.

Why shouldn't collections of particles act like whiffs of smoke, that separate upon contact with large particles? Indeed, how do particles themselves cohere? Such problems were real, and Descartes' physics was abandoned over the course of the eighteenth century. Nonetheless, it provided a conception for a comprehensive replacement of Aristotelian physics that persisted in the Newtonian vision of a unified physics of the celestial and terrestrial realms, and that continued in the mechanistic vision of life that was revived in the latter part of the nineteenth century.

This was especially true for what came to be known as the secondary qualities in the terminology of Robert Boyle and John Locke. The secondary qualities include colors, sounds, odors, tastes, and tactile qualities such as hot and cold. A red thing possesses the quality red in just the same way it possesses a shape: He rendered light as a property of particles and their motions: When light strikes an object, the particles that constitute light alter their rotation about their axis.

When particles with one or another degree of spin interact with the nerves of the retina, they cause those nerves to jiggle in a certain way.

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This jiggling is conveyed to the brain where it affects the animal spirits, which in turn affect the mind, causing the mind to experience one or another color, depending on the degree of spin and how it affects the brain. Color in objects is thus that property of their surface that causes light particles to spin in one way or another, and hence to cause one sensation or another. There is nothing else in the surface of an object, as regards color, than a certain surface-shape that induces various spins in particles of light.

Descartes introduced this new theory of sensory qualities in the first six chapters of the World. There, he defended it by arguing that his explanation of qualities in bodies in terms of size, shape, and motion are clearly understood by comparison with the Aristotelian qualities Subsequently, in the Meditations and Principles , he defended this account by appeal to the metaphysical result that body possesses only geometrical modes of extension. Real qualities are ruled out because they are not themselves instances of size, shape, or motion even if patches of color have a size and a shape, and can be moved about.

In addition to a new theory of sensory qualities, Descartes offered theories of the way in which the spatial properties—size, shape, distance, and position—are perceived in vision. It had been an area of inquiry since antiquity. Euclid and Ptolemy had each written on optical problems. During the Middle Ages, the Arabic natural philosopher Ibn al-Haytham produced an important new theoretical work in which he offered an extensive account of the perception of spatial properties.

The theoretical terrain in optics changed with Kepler's doctrine that vision is mediated by the retinal image and that the retina is the sensitive body in the eye. Descartes accepted Kepler's result and framed a new theory of spatial perception. Some of his theorizing simply adapted Ibn al-Haytham's theories to the newly discovered retinal image. Thus, Ibn al-Haytham held that size is perceived by combining the visual angle that a body subtends with perception of its distance, to arrive at a perception of the true size of the object. Visual angle is formed by the directions from a vantage point to a seen-object for a given fixation, e.

In al-Haytham's scheme, visual angle is registered at the surface of the crystalline humor. Descartes held that size is perceived by combining visual angle with perceived distance, but he now treated visual angle as the extent of an object's projection onto the retina. In Ibn al-Haytham's account, if the size of an object is known distance may be perceived through an inference; for a given size, an object's distance is inversely proportional to its visual angle. Descartes recognized this traditional account, depending as it does on past experience of an object's size and on an inference or rapid judgment that combines perceived visual angle with known or remembered size.

Descartes held that these rapid judgments are habitual and happen so quickly that they go unnoticed. Further, the sensations that present the objects in accordance with visual angle also go unnoticed, as they are rapidly replaced by visual experiences of objects at a distance. Ibn al-Haytham also explained that distance can be perceived by an observer's being sensitive to the number of equal portions of ground space that lie between the observer and a distant object.

Descartes did not adopt this explanation.

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However, Descartes used his mechanistic physiology to frame a new account of how distance might be perceived, a theory different from anything that could have been found in Ibn al-Haytham. In Kepler's new theory of how the eye works, an image is formed on the retina as a result of refraction by the cornea and lens. For objects at different distances, the focal properties of the system must be changed, just as the focal length of a camera is changed.

He then theorized that this change in the shape of the lens must be controlled by muscles, which themselves are controlled by nerve processes in the brain. Descartes realized that the central nervous state that controls accommodation would vary directly in proportion to the distance of objects. However, unlike the case of inferring distance from known size and visual angle, Descartes did not suppose that the mind is aware of the apparatus for controlling the accommodation of the eye. Rather, he supposed that, by an innate mechanism, the central brain state that varies with distance directly causes an idea of distance in the mind 6: This physiologically produced idea of distance could then be combined with perceived visual angle in order to perceive an object's size, as in al-Haytham's theory of size perception.

When we correctly perceive the distance and combine it with visual angle by an unnoticed mental act , the result is a veridical perception of a size-at-a-distance. Also, in saying an object ten times farther away than a near object should be a hundred times smaller, he is speaking of area; it would be ten times smaller in linear height. Descartes' work on visual perception is but one instance of his adopting a naturalistic stance toward conscious mental experience in seeking to explain aspects of such experience.

The Passions constitute another. It is sometimes said that Descartes' dualism placed the mind outside nature by rendering it as an immaterial substance. In this way, Descartes and his followers posited the existence of psychophysical or psychophysiological laws, long before Gustav Fechner —87 formulated a science of psychophysics in the nineteenth century. The things that readers find valuable in Descartes' work have changed over the centuries. We have seen that his natural philosophy had an immediate impact that lasted into the eighteenth century.

His theory of vision was part of that heritage, as were his results in mathematics. We have also seen that his mechanistic account of the psychology of the sensitive soul and his view that animals are like machines were revived in the nineteenth century. The fortune of the metaphysical and epistemological aspects of Descartes' philosophy is complex. In his own time, he inspired a raft of followers, who sought to develop his metaphysics, epistemology, natural philosophy, and even to add a worked-out ethics.

The British philosopher Henry More at first followed Descartes but subsequently turned against him. Other major philosophers, including Benedict de Spinoza and G. Leibniz, were influenced by Descartes' thought but developed their own, distinct systems. Perhaps the most profound effect that Descartes had on early modern epistemology and metaphysics arose from his idea to examine the knower as a means to determine the scope and possibilities of human knowledge. Among his immediate followers, Malebranche most fully developed this aspect of Descartes' philosophy.

Subsequent philosophers who were not followers of Descartes also adopted the strategy of investigating the knower. These authors came to different conclusions than had Descartes concerning the ability of the human mind to know things as they are in themselves. Hume and Kant especially—and each in his own way—rejected the very notion of a metaphysics that reveals reality as it is in itself. They did not merely deny Descartes' particular metaphysical theories; they rejected his sort of metaphysical project altogether.

But they did so through the type of investigation that Descartes himself had made prominent: During the twentieth century, various aspects of Descartes' philosophy were widely invoked and perhaps just as widely misinterpreted.