René Descartes (Portrait by Frans Hals, 1649)

René Descartes (1596 - 1650) was a French philosopher, mathematician, scientist and writer of the Age of Reason. He has been called the "Father of Modern Philosophy", and much of subsequent Western philosophy can be seen as a response to his writings. He is responsible for one of the best-known quotations in philosophy: "Cogito, ergo sum" ("I think, therefore I am").

He was a pioneer and major figure in 17th Century Continental Rationalism (often known as Cartesianism) later advocated by Baruch Spinoza and Gottfried Leibniz, and opposed by the British Empiricist school of thought of Hobbes, Locke, Berkeley and Hume. He represents a major break with the Aristotelianism and Scholasticism of the Medieval period.

His contribution to mathematics was also of the first order, as the inventor of the Cartesian coordinate system and the founder of analytic geometry, crucial to the invention of calculus and mathematical analysis. He was also one of the key figures in the scientific revolution of the 16th and 17th Centuries.

Descartes (pronounced day-CART) was born in the town of La Haye en Touraine (since renamed Descartes) in the Loire Valley in central France on 31 March 1596. His father, Joachim Descartes, was a busy lawyer and magistrate in the High Court of Justice, and his mother, Jeanne (née Brochard), died of tuberculosis when René was just one year old. René and his brother and sister, Pierre and Jeanne, were therefore mainly raised by their grandmother.

From 1604 until 1612, he attended the Jesuit Collège Royal Henry-Le-Grand at La Flèche, Anjou, studying classics, logic and traditional Aristotelianism philosophy. His health was poor and he was granted permission to remain in bed until 11 o'clock in the morning, a custom he maintained for the rest of his life. He then spent some time in Paris studying mathematics, before studing law at the University of Poitiers, in accordance with his father's wishes that he should become a lawyer, obtaining his law degree in 1616.

However, he then abandoned his education and spent several years travelling and experiencing the world (he later claimed that his formal education provided little of substance). It was during this time (in 1618) that he met the Dutch philosopher and scientist Isaac Beeckman (1588 - 1637) while walking through Breda in Holland, who sparked his interest in mathematics and the new physics.

In 1622, he returned to France, and soon afterwards sold all his property at La Haye, investing the proceeds in bonds which provided him with a comfortable income for the rest of his life. He returned to settle in Holland in 1628. The next year, he joined the University of Franeker; the year after that, Leiden University; and, in 1635, he is recorded as attending Utrecht University. He had a daughter, Francine, after a relationship in Amsterdam with a servant girl, Helène Jans, although Francine died at the age of fve. In fact, in the years between 1828 and 1649, he lived at 14 separate addresses in 10 different Dutch cities.

It was during this 20 year period of frequent moves that he wrote almost all of his major works on philosophy, mathematics and science. He shrewdly held off publication of his first work, "Le Monde" ("The World"), written between 1629 and 1633, due to the condemnation of the works of Galileo Galilei (1564 - 1642) and Nicolaus Copernicus (1473 - 1543) by the Roman Catholic Church in 1633. The most famous of his works include: the "Discours de la méthode pour bien conduire sa Raison et chercher la Vérité dans les Sciences" ("Discourse on the Method") of 1637, his first rationalist vision of the progress of human knowledge; the "Meditationes de Prima Philosophia" ("Meditations on First Philosophy") of 1641, a more formal exposition of his central tenets, in Latin; and the "Principia Philosophiae" ("Principles of Philosophy") of 1644, an even more systematic and comprehensive exposition of his views. For a time, in 1643, Cartesian philosophy was condemned by the University of Utrecht.

Descartes died of pneumonia on 11 February 1650 in Stockholm, Sweden, where he had been invited as a teacher for Queen Christina of Sweden. Later, his remains were taken to France and buried in the church of Sainte-Geneviève-du-Mont in Paris, and then, during the French Revolution, disinterred for burial in the Panthéon among the other great thinkers of France. Currently, his tomb is in the church of Saint-Germain-des-Prés in Paris, and his brain is in the Musée de l'Homme.

At the heart of Descartes' philosophical method is his refusal to accept the authority of previous philosophers, and even of the evidence of his own senses, and to trust only that which is clearly and distinctly seen to be beyond any doubt (a process often referred to as methodological skepticism or Cartesian doubt or hyperbolic doubt). Only then did he allow himself to reconstruct knowledge (piece by piece, such that at no stage is the possibility of doubt allowed to creep back in) in order to acquire a firm foundation for genuine knowledge and to dispel the Skepticism.

He outlined four main rules for himeself in his thinking:

• Never accept asnything except clear and distinct ideas.
• Divide each problem into as many parts are needed to solve it.
• Order your thoughts from the simple to the complex.
• Always check thoroughly for oversights.

In this process, detailed in his epochal "Discourse on the Method" of 1637 and expanded in the "Meditations on First Philosophy" of 1641, he first identified a single indubitable principle: that thought exists. He then argued that, if someone is wondering whether or not he exists, that is in and of itself proof that he does exist: "Je pense, donc je suis" ("I think, therefore I am") - the similar statement in Latin, "Cogito ergo sum" is found in his later "Principles of Philosophy". He realized that he could doubt whether he had a body (it could be that he was just dreaming of it or that it was an illusion created by an evil demon), but he could not doubt whether he had a mind, which suggested to him that the mind and body must be different things).

Descartes dismissed the senses and perception as unreliable (using the Wax Argument, that although the senses suggest that wax is a different thing when melted, it is in fact still wax), and concluded that reason and deduction is the only reliable method of attaining knowledge, which is the essence of Rationalism.

Descartes further argued that sensory perceptions come to him involuntarily (not willed by him), and are therefore external to his senses and therefore evidence of the existence of an external world outside of his mind. He argued that the things in the external world are material because God would not deceive him as to the ideas that are being transmitted, and has given him the propensity to believe that such ideas are caused by material things. Because of this belief that God is benevolent and does not desire to deceive him, he can therefore have some faith in the account of reality his senses provide him.

Descartes believed that the human body works like a machine, that it has the material properties of extension and motion, and that it follows the laws of physics. The pieces of the human machine, he argued, are like clockwork mechanisms, and that the machine could be understood by taking its pieces apart, studying them, and then putting them back together to see the larger picture (an idea referred to as Reductionism). The mind or soul, on the other hand, is a non-material entity that lacks extension and motion, and does not follow the laws of physics.

Descartes was the first to formulate the mind-body problem in the form in which it exists today, and the first to clearly identify the mind with consciousness and self-awareness, and to distinguish this from the brain, which was the physical seat of intelligence (Dualism). His particular form of Dualism proposed that the mind controls the body, but that the body also influences the otherwise rational mind (such as when people act out of passion), a two-way interaction which he claimed occurred in the pineal gland. This kind of Cartesian Dualism set the agenda for philosophical discussion of the mind-body problem for many years after Descartes' death (see the section on Philosophy of Mind).

It should be noted, however, that for all Descartes' innovation and boldness, he does not abandon the traditional idea of God. He uses his own variations of the causal argument, the ontological argument and the cosmological argument for the existence of God in his "Meditations" (see the section on Philosophy of Religion), and the existence of God plays a major role in his validation of reason and in other parts of Descartes’ system. Given the important rôle God plays in his work, suggestions that Descartes was really an atheist, and that he includes the arguments for the existence of God as window dressing, appear unlikley.

In mathematics, Descartes realized that a graph could be drawn to show a geometrical interpretation of a mathematical function using points known as Cartesian coordinates, and thereby founded analytic geometry or Cartesian geometry (using algebra to describe geometry), which was crucial to the subsequent development of calculus by Sir Isaac Newton (1643 - 1727) and Gottfried Leibniz. He also invented the notation which uses superscripts to indicate powers or exponents, and his rule of signs is also a commonly used method to determine the number of positive and negative zeros of a polynomial. It can be argued that his reflections on mind and mechanism, impelled by the invention of the electronic computer and by the possibility of machine intelligence, blossomed into the Turing test of a machine's capability to demonstrate intelligence.

In optics, he showed by using geometric construction and the law of refraction (also known as Descartes' law) that the angular radius of a rainbow is 42 degrees. He also independently discovered the law of reflection (that the angle of incidence equals the angle of reflection).

In physics, Descartes introduced (before Newton) the concept of momentum of a moving body (what he termed the "amount of motion"), which he defined as the product of the mass of the body and its velocity or speed. His three "laws of nature" became the basis of Newton's later laws of motion and the modern theory of dynamics: that each thing tries to remain in the same state and, once moved, continues to move; that all movement is along straight lines; and that when a body comes into contact with another body the combined "quantity of motion" remain the same (his conservation of motion principle).

In an attempt to explain the orbits of planets, Descartes also constructed his vortex theory which would become the most popular theory of planetary motion of the late 17th Century (although subsequently discredited). However, he continued to cling to the traditional mechanical philosophy of the 17th Century, which held that everything physical in the universe to be made of tiny "corpuscles" of matter (although, unlike Atomism, the theory maintained that there could be no vacuum, just a mass of swirling matter).