the philosophical and scientific traditions derived from the writings of the French philosopher René Descartes (Descartes, René) (1596–1650).

Metaphysically (metaphysics) and epistemologically, Cartesianism is a species of Rationalism, because Cartesians hold that knowledge—indeed, certain knowledge—can be derived through reason from innate ideas (innate idea). It is thus opposed to the tradition of Empiricism, which originated with Aristotle (384–322 BC), according to which all knowledge is based on sense experience and is therefore (because sense experience is fallible) only probable. In practice, however, Cartesians developed probabilistic scientific theories from observation and experiment, as did empiricists. Cartesians were forced to satisfy themselves with uncertainty in science (science, philosophy of) because they believed that God is omnipotent and that his will is entirely free; from this it follows that God could, if he so wished, make any apparent truth a falsehood and any apparent falsehood—even a logical contradiction—a truth. The human intellect, by contrast, is finite; thus, humans can be certain only of what God reveals and of the fact that they and God exist. Descartes argues that one has certain knowledge of one's own existence because one cannot think without knowing that one exists; this insight is expressed as “ cogito, ergo sum” (Latin: “I think, therefore I am”) in his Discourse on Method (1637) and as “I think, I am” in his Meditations (1641). In the Meditations, Descartes also argues that because we are finite, we cannot generate an idea of infinity, yet we have an idea of an infinite God, and thus God must exist to cause us to have that idea. He also says that although we have no direct acquaintance with the material world, not even with our own bodies, but only with ideas that represent the material world, we cannot know the material world directly. We know it exists only because God is not a deceiver.

Cartesians adopted an ontological dualism of two finite substances, mind (spirit or soul) and matter. The essence of mind is self-conscious thinking; the essence of matter is extension in three dimensions. God is a third, infinite substance, whose essence is necessary existence. God unites minds with bodies to create a fourth, compound substance, man. Humans obtain general knowledge by contemplating innate ideas of mind, matter, and God. For knowledge of particular events in the world, however, humans depend on bodily motions that are transmitted from sense organs through nerves to the brain to cause sensible ideas—i.e., sensations—in the mind. Thus, for Cartesians, knowledge of the material world is indirect.

This dualism of mind and matter gives rise to serious problems concerning causal interaction and knowledge. Given that mind and matter are so radically different, how can the body cause the mind to have sensible ideas? Likewise, how can the mind cause the body to move? How can the mind know the material world by way of sensible ideas, which are mental? In other words, how can ideas represent the properties of material objects, given that mind and matter are essentially distinct? Various lines of Cartesian philosophy developed from different answers to these questions.

Descartes's philosophy is rooted in his mathematics. He invented analytic geometry—a method of solving geometric problems algebraically and algebraic problems geometrically—which is the foundation of the infinitesimal calculus developed by Sir Isaac Newton (Newton, Sir Isaac) (1642–1727) and Gottfried Wilhelm Leibniz (Leibniz, Gottfried Wilhelm) (1646–1716). The method discussed in his Discourse on Method is basically an extension of analytic mathematical method, which he applies to all branches of science.

The first Cartesians were Dutch and French physicists and physiologists who attempted to explain physical and biological phenomena solely in mechanistic terms—i.e., solely in terms of matter and its motion and especially without appeal to Aristotelian notions such as form and final cause. Descartes's first disciple in the Netherlands, Henricus Regius (1598–1679), taught Cartesian physics at the University of Utrecht—though, to Descartes's chagrin, he dismissed Descartes's metaphysics as irrelevant to science. Another disciple, the French theologian and philosopher Nicolas Malebranche (Malebranche, Nicolas) (1638–1715), believed with Descartes that animals are merely machines and thus incapable of thought or feeling; he is said to have kicked a pregnant dog and then to have chastised critics such as Jean de La Fontaine (La Fontaine, Jean de) (1621–95), the French writer of animal fables, for expending their emotions over such inconsiderable creatures rather than concerning themselves with human misery. In Paris, the lectures of Pierre-Sylvain Régis (1632–1707) on Cartesian physics—which he accompanied with spectacular demonstrations of physical phenomena such as optical illusions—created such a sensation that Louis XIV forbade them. Because Cartesianism challenged the traditional Aristotelian science, which was supported by the Roman Catholic church, and because the church also stood behind the so-called “divine right” of kings to rule, the king feared that any criticism of traditional authority might give rise to revolution. (Later, in the 18th century, Descartes's emphasis on the ability of each individual to think for himself lent support to the cause of republicanism.)

Advancements in mechanical arts and crafts provided the practical foundation of Cartesian mechanism. In the 17th century, mechanical inventions such as statues that walked and talked by application of levers and pullies and organs that played by waterpower were well known. The mathematician Blaise Pascal (Pascal, Blaise) (1623–62) invented a calculating machine based on principles worked out by clock makers and inventors of spinning and knitting machines, such as the Englishman William Lee (Lee, William). The first inventors directly inspired by Descartes were the French craftsman Jean Ferrier, who attempted to make hyperbolic lenses according to Descartes's designs, and Étienne de Villebressieu, who with Descartes's collaboration developed an improved water pump.

Mechanism was promoted by one of Descartes's contemporaries, the mathematician and philosopher Marin Mersenne (Mersenne, Marin) (1588–1648). Pierre Gassendi (Gassendi, Pierre) (1592–1655) attempted to derive it theoretically from the atomism of the ancient Greek philosopher Epicurus (341–270 BC), who held that reality is ultimately constituted of “atoms” in motion in the “void.” Motion was first studied scientifically by the Italian mathematician and astronomer Galileo (1564–1642).

According to Descartes, the material universe (Cosmos) consists of an indefinitely large plenum of infinitely divisible matter, which is separated into the subtle matter of space and the denser matter of bodies by a determinate quantity of motion that is imparted and conserved by God. Bodies swirl like leaves in a whirlwind in vortices as great as that in which the planets sweep around the Sun and as small as that of tiny spinning globes of light. All bodily joinings and separations are mechanical, resulting from the collisions of other moving bodies. Because the amount of motion is conserved according to the laws of nature, the Cartesian material world exhibits a kind of determinism. After the initial impulse, the world evolves lawfully. If the speeds and positions of all the whirling portions of matter in the universe at any one moment could be completely described, then a complete description of their speeds and positions at any later time could be deduced through calculations based on the laws of motion. Of course, only God has the infinite intellect required for performing these calculations.

Although God is the primary cause of the existence of the material universe and of the laws of nature, all physical events—all movements and interactions of bodies—result from secondary causes—that is, from bodies colliding with each other. God stands merely for the uniformity and consistency of the laws of nature. This led Blaise Pascal (Pascal, Blaise) to complain that the only purpose God serves in Descartes's system is to initiate motion in the material world and to guarantee its conservation and the uniformity of nature.

Cartesianism was criticized in England by the Platonist philosopher Henry More (More, Henry) (1614–87) and was popularized by Antoine Le Grand (1629–99), a French Franciscan, who wrote an exposition of the Cartesians' ingenious account of light and colour. According to popular versions of this account, light consists of tiny spinning globes of highly elastic subtle matter that fly through the air in straight lines and bounce like balls at angles consistent with the optical laws of reflection and refraction. Different colours are caused by the globes' different speeds and spins, which themselves are determined by the texture of the surfaces on which the globes are reflected, refracted, or transmitted. The spectrum of colours observed when light passes through a triangular prism is explained by the fact that the globes pass more slowly through thicker parts of the prism than they do through thinner ones. The same spectrum of colours occurs when light passes through thicker and thinner parts of raindrops, giving rise to rainbows. Although Newton (Newton, Sir Isaac) and Leibniz (Leibniz, Gottfried Wilhelm) later showed that the simple mechanistic principles underlying these accounts were incapable of explaining the forces of gravitation and chemical bonding, it is noteworthy that the Cartesian theory of light is similar in principle to the contemporary view, according to which the different colours are produced by light at different wavelengths.

By the end of the 17th century, most of Cartesian physics had been superseded by Newtonian mathematical physics. Cartesians admitted that Descartes's laws of motion were wrong and that his principle of the conservation of motion should be abandoned in favour of Newton's principles of the conservation of energy (energy, conservation of), or vis viva (Latin: “living force”), and linear momentum. Although the Treatise (1671) of Jacques Rohault, a leading expositor of Cartesian physics, was translated into English in 1723 by Newton's disciple Samuel Clarke (Clarke, Samuel) (1675–1729) and Clarke's brother, their corrections and annotations turned the work into an exposition of Newtonian physics. Nevertheless, this progress would have pleased Descartes, who said that the advancement of scientific knowledge would take centuries of work.

Cartesian mechanism was opposed to scholastic (Scholasticism) Aristotelian science, which was supported by both Roman Catholic and Protestant theologians. These thinkers held that, because all things are created by God with a given nature, there can be no evolutionary development of animals or of the universe as a whole. For Aristotle, all living things possess a spirit or “ soul,” which is the form, or organizing principle, of the matter out of which the organism is composed, as well as the source of its powers of growth and development, nutrition, perception, and (in humans) cognition. The soul is the essence, or nature, of the organism and its final cause—i.e., its purpose, or goal. Thus, the development of an acorn into an oak tree is explained by the fact that the acorn possesses a form that directs it toward this end.

Descartes rejected both the teleological, animistic view and the related theory of alchemy that there are vital forces in things. Cartesians denied the existence of what they considered occult (occultism) or magical (magic) forces, insisting instead that only God and humans have spirits, wills, purposes, and ends. They conceived both animate and inanimate bodies as having no goals but as simply being pushed around passively. For Cartesians, science therefore consisted of looking not for final causes but rather for the laws that govern the motions of bodies.

By insisting on human free will, Descartes placed the human soul or mind, like God, outside deterministic nature. Because the body is a part of nature, however, the mind's evident ability to control the body's movements is, on Cartesian assumptions, inexplicable and miraculous and thus inconsistent with mechanistic determinism. Ironically, in Descartes's system this ability is itself an occult or magical force.

Most Cartesians believed that the mind and body interact. When asked how this is possible, Régis gave the standard Cartesian reply: human beings experience the interaction, and God can and does make it take place, even if we cannot understand how. As for the question of how ideas represent objects, Rohault spoke for all Cartesians when he asserted that God can make ideas represent material bodies without resembling them; no further explanation is necessary. In both of these replies, the Cartesians can be seen to abdicate philosophy for mysticism and theology.

According to the Thomists (Thomism) (adherents of the Aristotelian philosophical and theological system developed by St. Thomas Aquinas (Aquinas, Thomas, Saint)), the soul or mind is the form of the body. Although for Aristotle the form of an object is inseparable from the matter of which it is made, the Thomists held that the human soul is a “substantial form” that is miraculously able to exist independently of matter and thus to survive the death of the body. Descartes, by contrast, contended that the notion of substantial form is contradictory, because it assumes the separate existence of something that by definition can exist only in unity with matter. For Cartesians, the mind or soul is a substance existing in itself, independently of matter; thus, they were able to explain immortality without having to rely on the dubious assumption that the soul-form is a kind of substance. This view, however, creates a serious problem concerning the ultimate nature of human beings. According to Cartesians, sensible ideas arise from the union of mind and body for the sole purpose of preserving the body by presenting harmful things as painful and beneficial things as pleasurable. Human beings learn by experience what to seek and to avoid, and the memory of these experiences is preserved in the brain. Once the body dies, however, both the need for sensible ideas and their memory traces in the brain are destroyed. All the soul knows of matter after death is the general idea of extension. Because all bodily associations and memories are eliminated, however, individual personality is lost; each human being survives death only as an impersonal soul, identical to all other bodiless souls. Like the notion that animals are mere machines, the Cartesian conclusion that the sensible manifestations of this life are neither continued nor remembered in the next was unpopular.

In addition to the dualism of mind and matter in Cartesian metaphysics, there is a more general dualism in Cartesianism as a whole between a rationalist metaphysics and epistemology, which entails the existence of innate ideas of mind, matter, and God and the possibility of obtaining certain knowledge through reason, and an empiricist (and mechanistic) physics, according to which scientific knowledge, which is never certain, is gradually accumulated through observation and experience of the material world. Descartes's insistence on the possibility of certain knowledge of God's existence has led some commentators to present him primarily as an apologist for Christianity. Others, however, have argued that he was really an atheist and a materialist who made arguments for God's existence only to protect himself from persecution by the church.

Although Descartes publicly denied an interest in theology, in letters he offered mechanistic explanations of transubstantiation. According to the Thomistic account of this mystery, the forms of bread and wine are miraculously sustained as substantial forms while their matter is replaced by Christ's flesh and blood. Rohault appealed to the Cartesian view that sensible ideas are caused by configurations of the parts of material bodies to argue that, if bread and wine were replaced by flesh and blood whose parts had exactly the same configurations, the flesh and blood would look, feel, and taste like bread and wine. Although Rohault's account still requires the miraculous replacement of bread and wine by flesh and blood, it does not rely on the self-contradictory notion of substantial form.

A deterministic Cartesian ethics was developed by the Flemish Calvinist philosopher Arnold Geulincx (Geulincx, Arnold) (1624–69). In his view, although one can do only what God has willed, one is free to accept what one must do willingly or unwillingly. Virtue consists in the humble, diligent, and obedient acceptance of the justice of God's decrees in the light of reason, whereas sin and evil result from an egotistic (and futile) stand against God. This Stoic ethics, with its affinity to Calvinist and Jansenist predestinarianism, is as deterministic as Cartesian physics. It does, however, contradict Descartes's claim that the human will is free not just to accept or reject the rightness of predetermined bodily actions but also to choose and cause particular actions.

The most important philosophical work stemming directly from Descartes's writings is The Search After Truth (1674–75), by Malebranche. His position, known as occasionalism, was adopted also by Geulincx and the French philosopher Géraud de Cordemoy (Cordemoy, Géraud de). Malebranche was convinced by the argument—urged most strongly by the French skeptic Simon Foucher (Foucher, Simon)—that, because they are so radically different, Cartesian mind and matter cannot interact. Malebranche held that, on every occasion when human bodies interact with the world, God provides the appropriate sensible ideas in human minds. And, on every occasion when human minds will that their bodies move, God makes them move. Thus, there is no direct causal interaction between mind and body; there are only separate but parallel sequences of mental and material events intermediated by God.

Foucher also argued that, because sensible ideas cannot resemble material things, they cannot represent them either, and they thus cannot be a source of knowledge of the material world. In other words, because sensible ideas such as colours, tactile feelings, sounds, odours, and tastes—as they are experienced by the mind—are utterly unlike the properties of material bodies, which are limited to size, shape, position, and motion or rest, it follows that these ideas cannot give knowledge of the material world as it really is. In response, Malebranche, like Descartes before him, simply denied that ideas must resemble their objects to represent them. Regarding the possibility that one might have sensible ideas of a nonexistent world, Malebranche said tersely that the first chapter of Genesis assures the existence of the material world. As to how man's ideas of this world are true, Malebranche offered the Platonic view that ideas of all things reside in God and that, on appropriate occasions, God illuminates these ideas for human observation. Thus, human beings see all things in God and can rest assured in his goodness.

Despite Descartes's influence on his thought, Malebranche denied that he was a Cartesian. Unlike Descartes, he argued that introspection gives no knowledge of the essence of the mind. This view prompted the English empiricist philosopher John Locke (Locke, John) (1632–1704) to suggest that, for all human beings know, matter might be able to think. All Cartesians opposed this possibility, however, because the essential difference between mind or soul and body supports the Christian doctrine that the human soul survives the body's death.

The rationalist metaphysics of the Dutch-Jewish philosopher Benedict de Spinoza (Spinoza, Benedict de) derives from Descartes. Spinoza wrote his Ethics (1677) in mathematico-deductive form, with definitions, axioms, and derived theorems. His metaphysics, which is simultaneously monistic, pantheistic, and deistic, holds that there is only one substance, that this one substance is God, and that God is the same as the world. The one substance has an infinite number of attributes, each of which expresses the totality of the world (or God), though the only attributes known to human beings are mind and matter. All attributes are parallel (psychophysical parallelism) in every respect; that is, for every idea expressed in the mental attribute, there is a parallel body in the material attribute, and vice versa. Thus, though mind and matter do not interact, for Spinoza as for Malebranche they appear to do so.

The other great figure of late 17th-century rationalism, Leibniz (Leibniz, Gottfried Wilhelm), also gave a parallelistic answer to the problem of mind-body interaction. Leibniz asserted that the universe is constituted of an infinite number of simple, indivisible, extensionless substances, which he called “monads (monad).” Each monad reflects, or perceives, the entire universe from its own point of view. Although monads do not causally interact with each other, a “preestablished harmony” between them, created and maintained by God, ensures that the appearance of interaction is maintained at the level of material objects.

The Irish radical empiricist and bishop George Berkeley (Berkeley, George) (1685–1753) developed another monistic metaphysical system. Berkeley managed to avoid the problem of mind-body interaction by taking the extreme step of denying the existence of matter. Bodies, according to him, are only collections of sensible ideas that are presented to the human mind in lawful order by God. Because there is no material world, there is also no skeptical problem about whether ideas truly represent physical reality. Instead, all ideas are known directly.

By contrast, the English materialist philosopher Thomas Hobbes (Hobbes, Thomas) (1588–1679) did away with mind as a mental substance by asserting that only matter exists. For Hobbes, the mind is the same as the brain, and thoughts or ideas consist of nothing more than motions of brain matter. Because the mind is material, it is capable of causing bodily motions in response to sensory stimuli; and because ideas are material, they can resemble, and thus represent, material bodies.

Two important themes in the history of modern philosophy can be traced to Descartes. The first, called “the way of ideas,” represents the attempt in epistemology to provide a foundation for our knowledge of the external world (as well as our knowledge of the past and of other minds) in the mental experiences of the individual. The Cartesian theory of knowledge through representative ideas is rooted in Galileo's distinction between real, or primary, properties of material bodies—such as size, shape, position, and motion or rest—which were thought to exist in bodies themselves, and sensible, or secondary, properties—such as colours, tactile feelings, sounds, odours, and tastes—which were thought to exist only in the mind. As Descartes assumes in his theory of light and as Locke (Locke, John) later argued, secondary properties of bodies do not exist in bodies themselves but are the result of the interaction of distinctive arrangements of primary properties with the human sense organs. According to Locke, however, our sensible ideas of the size, shape, position, and motion or rest of particular bodies resemble their corresponding primary properties and so can be a source of knowledge about them. Nevertheless, against this claim it is still possible to raise the skeptical objection that, because mental and material substances are radically distinct, and because all ideas are mental, no idea, not even an idea of a primary property, can resemble a material object.

As noted above, Berkeley's phenomenalism is one heroic solution to this skeptical problem: Bodies are known directly simply because bodies are nothing more than bundles of sensible ideas. Another response, also heroic, is that of the Scottish philosopher David Hume (Hume, David) (1711–76), who accepted skeptical conclusions and contented himself with attempting to explain the psychological origins of our unjustifiable belief in an external world, in the continuity of past and future, and in an enduring “self” that is the unchanging subject of mental experience. Early in the 20th century, the British philosopher Bertrand Russell (Russell, Bertrand) (1872–1970) and his student the Austrian-born Ludwig Wittgenstein (Wittgenstein, Ludwig) (1889–1951), as well as the German founders of Logical Positivism Moritz Schlick (Schlick, Moritz) (1882–1936) and Rudolf Carnap (Carnap, Rudolf) (1891–1970), construed aspects of the physical world as “logical constructions” of sensible ideas, which they called “sense data.” The German philosopher Edmund Husserl (Husserl, Edmund) (1859–1939) attempted to establish a science of sensible ideas, which he called Phenomenology. Later in the century, Russell, following the American pragmatist philosopher and psychologist William James (James, William) (1842–1910), suggested that both mind and matter could be constructed out of what he called “neutral monads. (neutral monism)” All of these systems can be considered steps along the Cartesian way of ideas.

The second theme to derive from Descartes is an emphasis on the nature of the self, or ego. The roots of this idea extend back to the Neoplatonic philosophy of St. Augustine (Augustine, Saint) (354–430), who argued that when one is thinking, one necessarily exists. The idea also was central to the developmental idealism of the German philosopher G.W.F. Hegel (Hegel, Georg Wilhelm Friedrich) (1770–1831), who conceived of human history as the gradual coming to consciousness of a World Soul. The metaphysics of Martin Heidegger (Heidegger, Martin) (1889–1976), with its focus on the being of the self, or Dasein, strongly influenced the Existentialism of the French philosopher Jean-Paul Sartre (Sartre, Jean-Paul) (1905–1980), who argued that each individual chooses his own nature. Sartre also upheld the Cartesian position that the self is essentially conscious by rejecting the theory of the unconscious proposed by the Austrian psychoanalyst Sigmund Freud (Freud, Sigmund) (1856–1939).

Some aspects of Cartesian metaphysics and epistemology were still strongly defended in the 20th century. The American linguist Noam Chomsky (Chomsky, Noam), for example, has argued that human beings are born with an innate knowledge of the underlying structures of all learnable languages, even of languages that have never been spoken. The Nobel Prize-winning Australian physiologist John C. Eccles (Eccles, Sir John Carew) (1903–97) and the British primatologist Wilfred E. Le Gros Clark (1895–1971) developed theories of the mind as a nonmaterial entity. Similarly, Eccles and the Austrian-born British philosopher Karl Popper (Popper, Sir Karl) (1902–94) advocated a species of mind-matter dualism, though their tripartite division of reality into matter, mind, and ideas is perhaps more Platonic than Cartesian.

One of the strongest contemporary attacks on traditional Cartesian dualism is that of the British philosopher Gilbert Ryle (Ryle, Gilbert) (1900–76). In The Concept of Mind (1949), Ryle dismisses the Cartesian view as the fallacy of “the ghost in the machine,” arguing that the mind—the ghost—is really just the intelligent behaviour of the body. A different criticism has been advanced by the American pragmatist Richard Rorty (Rorty, Richard), who claims (in Philosophy and the Mirror of Nature 【1979】 and other works) that the Cartesian demand for certain knowledge of an objectively existing world through representative ideas is a holdover from the mistaken quest for God. That is, whereas certain knowledge of God's existence may be necessary for salvation, to seek certainty in science and in the ordinary affairs of life is both hopeless and unnecessary. Philosophy in the Cartesian tradition, Rorty contends, is the 20th century's substitute for theology and should, like the concept of God, be gently laid to rest.

In the late 20th and early 21st centuries, the nature of consciousness became a topic of particular interest to philosophers and neuroscientists. The problems faced by these researchers were essentially the same as those encountered by all philosophers since Descartes who have attempted to understand the nature of the mind. Although the seat of consciousness is universally accepted to be the central nervous system, and in particular the brain, it seems impossible that a material object like the brain could give rise to the mental experiences that human beings have when they are said to be conscious. In other words, it seems impossible to give an account of these experiences that, on the one hand, captures what they are really like for human beings and, on the other, is consistent with the strictly physical vocabulary of the scientific theories in terms of which the brain is understood.

Some philosophers have responded to this problem in a manner reminiscent of Descartes, who argued that, although mind-body interaction seems to be impossible, human beings experience it, and God can make it happen. The British philosopher Colin McGinn, for example, is among a group of thinkers, known as “mysterians,” who claim that, although we know that the conscious mind is nothing more than the brain, it is simply beyond the conceptual apparatus of human beings to understand how this can be the case. Other philosophers, such as Daniel Dennett and Paul Churchland, have made valiant attempts to develop strictly materialist accounts of consciousness, but their efforts so far have not been widely accepted. A third line of response is represented by the American philosopher John Searle, who argues that the root of the problem is the dichotomy between the old Cartesian concepts of mind and matter, which he claims are both inherently incompatible and outmoded, given modern physics. Searle believes that consciousness, like digestion, is a biological phenomenon (albeit a very complex one) that can in principle be fully explained in scientific terms.

Descartes's influence on Western philosophy is so pervasive that all Western philosophers, even those who reject Cartesianism, can be said to be Cartesians, just as they can be said to be Greeks: their positions are essentially responses to problems posed by Descartes. Descartes also stands at the beginning of modern mathematics through his contribution to the development of the infinitesimal calculus by Newton and Leibniz. Descartes's skeptical, mathematical method underpins modern science; his conception of rationality informed modern Western ideas of what it means to be a human being until nearly the end of the 20th century; and his intense desire to control nature in the service of humanity has been the ultimate secular goal of modern science since the time of the Enlightenment.