René Descartes (French IPA: [ʁə'ne de'kaʁt]) (March 31, 1596 – February 11, 1650), also known as Renatus Cartesius (latinized form), was a highly influential French philosopher, mathematician, scientist, and writer. He has been dubbed the "Father of Modern Philosophy" and the "Father of Modern Mathematics," and much of subsequent Western philosophy is a reaction to his writings, which have been closely studied from his time through the present day. His influence in mathematics is also apparent; the Cartesian coordinate system that is used in plane geometry and algebra being named for him and he was one of the key figures in the Scientific Revolution.
Descartes was a major figure in seventeenth century continental rationalism, later advocated by Baruch Spinoza and Gottfried Leibniz, and opposed by the empiricist school of thought consisting of Hobbes, Locke, Berkeley, and Hume. Leibniz, Spinoza, and Descartes were all versed in mathematics as well as philosophy, and Descartes and Leibniz contributed greatly to science as well. As the inventor of the Cartesian coordinate system, Descartes founded analytic geometry, the bridge between algebra and geometry crucial to the invention of calculus and analysis.
(New World Encyclopedia)
Descartes is often regarded as the first modern thinker to provide a philosophical framework for the natural sciences as they began to develop. He attempts to arrive at a fundamental set of principles that one can know as true without any doubt. To achieve this, he employs a method called methodological skepticism: he rejects any idea that can be doubted, and then reestablishes them in order to acquire a firm foundation for genuine knowledge. Initially, Descartes arrives at only a single principle: Thought exists. Thought cannot be separated from the thinker, therefore, the thinker exists (Discourse on the Method and Principles of Philosophy). Most famously, this is known as cogito ergo sum (Latin: "I think, therefore I am"), or more aptly, "Dubito, ergo cogito, ergo sum" (Latin: "I doubt, therefore I think, therefore I am"). Therefore, Descartes concluded, if he doubted, then something or someone must be doing the doubting, therefore the very fact that he doubted proved his existence.
Descartes concludes that he can be certain that he exists because he thinks. But in what form? He perceives his body through the use of the senses; however, these have previously been proven unreliable. So Descartes concludes that the only indubitable knowledge is that he is a thinking thing. Thinking is his essence as it is the only thing about him that cannot be doubted. Descartes defines "thought" (cogitatio) as "what happens in me such that I am immediately conscious of it, insofar as I am conscious of it." Thinking is, thus, every activity of a person of which he is immediately conscious.
To further demonstrate the limitations of the senses, Descartes proceeds with what is known as the Wax Argument. He considers a piece of wax: His senses inform him that it has certain characteristics, such as shape, texture, size, color, smell, and so forth. When he brings the wax towards a flame, these characteristics change completely. However, it seems that it is still the same thing: It is still a piece of wax, even though the data of the senses inform him that all of its characteristics are different. Therefore, in order to properly grasp the nature of the wax, he cannot use the senses: He must use his mind. Descartes concludes:
Thus what I thought I had seen with my eyes, I actually grasped solely with the faculty of judgment, which is in my mind.
In this manner, Descartes proceeds to construct a system of knowledge, discarding perception as unreliable and instead admitting only deduction as a method. In the third and fifth Meditation, he offers an ontological proof of a benevolent God (through both the ontological argument and trademark argument). Because God is benevolent, he can have some faith in the account of reality his senses provide him, for God has provided him with a working mind and sensory system and does not desire to deceive him; however, this is a contentious argument, as his very notion of a benevolent God from which he developed this argument is easily subject to the same kind of doubt as his perceptions. From this supposition, however, he finally establishes the possibility of acquiring knowledge about the world based on deduction and perception. In terms of epistemology therefore, he can be said to have contributed such ideas as a rigorous conception of foundationalism and the possibility that reason is the only reliable method of attaining knowledge.
In Descartes's system, knowledge takes the form of ideas, and philosophical investigation is the contemplation of these ideas. This concept would influence subsequent internalist movements, as Descartes's epistemology requires that a connection made by conscious awareness will distinguish knowledge from falsity. As a result of his Cartesian doubt, he sought for knowledge to be "incapable of being destroyed," in order to construct an unshakable ground upon which all other knowledge can be based. The first item of unshakable knowledge that Descartes argues for is the aforementioned cogito, or thinking thing.
Descartes also wrote a response to skepticism about the existence of the external world. He argues that sensory perceptions come to him involuntarily, and are not willed by him. They are external to his senses, and according to Descartes, this is evidence of the existence of something outside of his mind, and thus, an external world. Descartes goes on to show that the things in the external world are material by arguing that God would not deceive him as to the ideas that are being transmitted, and that God has given him the "propensity" to believe that such ideas are caused by material things.
Dualism
Descartes suggested that the body works like a machine, that it has the material properties of extension and motion, and that it follows the laws of physics. The mind (or soul), on the other hand, was described as a nonmaterial entity that lacks extension and motion, and does not follow the laws of physics. Descartes argued that only humans have minds, and that the mind interacts with the body at the pineal gland. This form of dualism proposes that the mind controls the body, but that the body can also influence the otherwise rational mind, such as when people act out of passion. Most of the previous accounts of the relationship between mind and body had been uni-directional.
Descartes suggested that the pineal gland is "the seat of the soul" for several reasons. First, the soul is unitary, and unlike many areas of the brain the pineal gland appears to be unitary (microscopic inspection reveals it is formed of two hemispheres). Second, Descartes observed that the pineal gland was located near the ventricles. He believed the animal spirits of the ventricles acted through the nerves to control the body, and that the pineal gland influenced this process. Finally, Descartes incorrectly believed that only humans have pineal glands, just as, in his view, only humans have minds. This led him to the belief that animals cannot feel pain, and Descartes's practice of vivisection (the dissection of live animals) became widely practiced throughout Europe until the Enlightenment.
Cartesian dualism set the agenda for philosophical discussion of the mind-body problem for many years after Descartes's death. The question of how a nonmaterial mind can influence a material body, without invoking supernatural explanations, remains an enigma to this day.
Modern scientists have criticized Cartesian dualism, as well as its influence on subsequent philosophers.
Mathematical and Scientific legacy
Descartes' theory provided the basis for the calculus of Newton and Leibniz, by applying infinitesimal calculus to the tangent line problem, thus permitting the evolution of that branch of modern mathematics.This appears even more astounding considering that the work was just intended as an example to his Discours de la méthode pour bien conduire sa raison, et chercher la verité dans les sciences (Discourse on the Method to Rightly Conduct the Reason and Search for the Truth in Sciences, better known under the shortened title Discours de la méthode).
Descartes' rule of signs is also a commonly used method in modern mathematics to determine possible quantities of positive and negative zeros of a function.
Descartes invented analytic geometry, and discovered the law of conservation of momentum. He outlined his views on the universe in his Principles of Philosophy.
Descartes also made contributions to the field of optics. He showed by using geometric construction and the law of refraction (also known as Descartes's law) that the angular radius of a rainbow is 42 degrees (that is, the angle subtended at the eye by the edge of the rainbow and the ray passing from the sun through the rainbow's center is 42 °). He also independently discovered the law of reflection, and his essay on optics was the first published mention of this law.
One of Descartes' most enduring legacies was his development of Cartesian geometry, the algebraic system taught in schools today. He also created exponential notation, indicated by numbers written in what is now referred to as superscript (such as x²).
Modern philosophy may be said to begin with Descartes’s Meditations, with the self in solitude, meditating, becoming conscious of the false and doubtful ideas one has accepted so far in life, and deciding that the time has come to overthrow all of one’s beliefs.
In the first sentences of the Meditations, Descartes says: “Everything must be thoroughly overthrown for once in my life, if I ever want to establish anything solid and permanent in the sciences”.
Can I, by my own reason, establish solid and permanent truth?
Descartes’s answer is that of Plato and all rationalism. Rationalism claims that reason is universal in all human beings: that reason is the most important element in human nature; that reason is the only means to certainty in knowledge; that reason is the only way to determine what is morally right and good and what constitutes a good society.
Descartes’s model for the use of reason is mathematics. In his Discourse on Method, Descartes says: “Of all who have sought for the truth in the sciences, it has been the mathematicians alone who have been able to succeed in producing reasons which are evident and certain”. And it was the method of mathematics, using reason alone, Descartes believes, which enabled Copernicus to revolutionize astronomy with his heliocentric theory of the universe, and enabled Galileo to provide the proof of the Copernican theory.
Mathematics, Descartes thinks, can clear up the confusions and uncertainties of philosophy. The method of mathematics will gain the same clarity and certainty for philosophy as for geometry, and as the scientists have gained for physics and astronomy.
Descartes tells us, in his Rules for the Direction of the Mind, that mathematics consists in the use of only two mental operations by which true knowledge can be achieved: intuition and deduction.
Intuition
By intuition he means our understanding of self-evident principles, such as the axioms of geometry or such as an arithmetic equation. These statements are self-evident in that they prove themselves to reason: To understand them is to know that they are absolutely true; no rational mind can doubt them.
Deduction. By deduction he means orderly, logical reasoning or inference from self-evident propositions, as all of geometry is reasoned in strict order by deduction from its self-evident axioms and postulates. “The chief secret of method,” says Descartes, “is to arrange all facts into a deductive, logical system.
Descartes’s goal as a philosophy is to build a system of philosophy based upon intuition and deduction which will remain as certain and as imperishable as geometry.
The entire series of the six meditations, day after day, is a single sustained effort to reconstruct philosophy, to find for philosophy the certainty of a mathematical proof.
Descartes lays down three requirements which this foundational belief must meet:
1. Its certainty must be such that it is impossible to doubt, it is self-evident to reason, it is an idea which is clear (in itself) and distinct (from every other belief).
2. Its certainty must be ultimate and not dependent upon the certainty of any other belief.
3. It must be about something which exists (so that from it beliefs about the existence of other things may be deduced).
In 1641, Descartes published the Meditations on First Philosophy.The Meditations is characterized by Descartes’s use of methodic doubt, a systematic procedure of rejecting as though false all types of belief in which one has ever been, or could ever be, deceived. His arguments derive from the skepticism of the Greek philosopher Sextus Empiricus (fl. 3rd century ce) as reflected in the work of the essayist Michel de Montaigne (1533–92) and the Catholic theologian Pierre Charron (1541–1603). Thus, Descartes’s apparent knowledge based on authority is set aside, because even experts are sometimes wrong. His beliefs from sensory experience are declared untrustworthy, because such experience is sometimes misleading, as when a square tower appears round from a distance. Even his beliefs about the objects in his immediate vicinity may be mistaken, because, as he notes, he often has dreams about objects that do not exist, and he has no way of knowing with certainty whether he is dreaming or awake. Finally, his apparent knowledge of simple and general truths of reasoning that do not depend on sense experience—such as “2 + 3 = 5” or “a square has four sides”—is also unreliable, because God could have made him in such a way that, for example, he goes wrong every time he counts. As a way of summarizing the universal doubt into which he has fallen, Descartes supposes that an “evil genius of the utmost power and cunning has employed all his energies in order to deceive me.”
Although at this stage there is seemingly no belief about which he cannot entertain doubt, Descartes finds certainty in the intuition that, when he is thinking—even if he is being deceived—he must exist. In the Discourse, Descartes expresses this intuition in the dictum “I think, therefore I am”; but because “therefore” suggests that the intuition is an argument—though it is not—in the Meditations he says merely, “I think, I am” (“Cogito, sum”). The cogito is a logically self-evident truth that also gives intuitively certain knowledge of a particular thing’s existence—that is, one’s self. Nevertheless, it justifies accepting as certain only the existence of the person who thinks it. If all one ever knew for certain was that one exists, and if one adhered to Descartes’s method of doubting all that is uncertain, then one would be reduced to solipsism, the view that nothing exists but one’s self and thoughts. To escape solipsism, Descartes argues that all ideas that are as “clear and distinct” as the cogito must be true, for, if they were not, the cogito also, as a member of the class of clear and distinct ideas, could be doubted. Since “I think, I am” cannot be doubted, all clear and distinct ideas must be true.
On the basis of clear and distinct innate ideas, Descartes then establishes that each mind is a mental substance and each body a part of one material substance. The mind or soul is immortal, because it is unextended and cannot be broken into parts, as can extended bodies. Descartes also advances at least two proofs for the existence of God. The final proof, presented in the Fifth Meditation, begins with the proposition that Descartes has an innate idea of God as a perfect being. It concludes that God necessarily exists, because, if he did not, he would not be perfect. This ontological argument for God’s existence, introduced by the medieval English logician St. Anselm of Canterbury (1033/34–1109), is at the heart of Descartes’s rationalism, for it establishes certain knowledge about an existing thing solely on the basis of reasoning from innate ideas, with no help from sensory experience. Descartes elsewhere argues that, because God is perfect, he does not deceive human beings, and therefore, because God leads us to believe that the material world exists, it does exist. In this way Descartes claims to establish metaphysical foundations for the existence of his own mind, of God, and of the material world.
The inherent circularity of Descartes’s reasoning was exposed by Arnauld, whose objection has come to be known as the Cartesian Circle. According to Descartes, God’s existence is established by the fact that Descartes has a clear and distinct idea of God; but the truth of Descartes’s clear and distinct ideas are guaranteed by the fact that God exists and is not a deceiver. Thus, in order to show that God exists, Descartes must assume that God exists.