Rene Descartes (; French: [R@ne dekaRt]; Latinized: Renatus Cartesius; adjectival form: "Cartesian"; 31 March 1596 - 11 February 1650) was a French philosopher, mathematician, and scientist. Dubbed the father of modern western philosophy, much of subsequent Western philosophy is a response to his writings, which are studied closely to this day. A native of the Kingdom of France, he spent about 20 years (1629-49) of his life in the Dutch Republic after serving for a while in the Dutch States Army of Maurice of Nassau, Prince of Orange and the Stadtholder of the United Provinces. He is generally considered one of the most notable intellectual representatives of the Dutch Golden Age.
Philosophical work
Rene du Perron Descartes was born in La Haye en Touraine (now Descartes, Indre-et-Loire), France, on 31 March 1596. His mother, Jeanne Brochard, died soon after giving birth to him, and so he was not expected to survive. Descartes' father, Joachim, was a member of the Parlement of Brittany at Rennes. Rene lived with his grandmother and with his great-uncle. Although the Descartes family was Roman Catholic, the Poitou region was controlled by the Protestant Huguenots. In 1607, late because of his fragile health, he entered the Jesuit College Royal Henry-Le-Grand at La Fleche, where he was introduced to mathematics and physics, including Galileo's work. After graduation in 1614, he studied for two years (1615-16) at the University of Poitiers, earning a Baccalaureat and Licence in canon and civil law in 1616, in accordance with his father's wishes that he should become a lawyer. From there he moved to Paris.  In his book Discourse on the Method, Descartes recalls,  I entirely abandoned the study of letters. Resolving to seek no knowledge other than that of which could be found in myself or else in the great book of the world, I spent the rest of my youth traveling, visiting courts and armies, mixing with people of diverse temperaments and ranks, gathering various experiences, testing myself in the situations which fortune offered me, and at all times reflecting upon whatever came my way so as to derive some profit from it.  Given his ambition to become a professional military officer, in 1618, Descartes joined, as a mercenary, the Protestant Dutch States Army in Breda under the command of Maurice of Nassau, and undertook a formal study of military engineering, as established by Simon Stevin. Descartes, therefore, received much encouragement in Breda to advance his knowledge of mathematics. In this way, he became acquainted with Isaac Beeckman, the principal of a Dordrecht school, for whom he wrote the Compendium of Music (written 1618, published 1650). Together they worked on free fall, catenary, conic section, and fluid statics. Both believed that it was necessary to create a method that thoroughly linked mathematics and physics.  While in the service of the Catholic Duke Maximilian of Bavaria since 1619, Descartes was present at the Battle of the White Mountain outside Prague, in November 1620.  Descartes apparently started giving lessons to Queen Christina after her birthday, three times a week, at 5 a.m, in her cold and draughty castle. Soon it became clear they did not like each other; she did not like his mechanical philosophy, nor did he appreciate her interest in Ancient Greek. By 15 January 1650, Descartes had seen Christina only four or five times. On 1 February he contracted pneumonia and died on 11 February. The cause of death was pneumonia according to Chanut, but peripneumonia according to the doctor Van Wullen who was not allowed to bleed him. (The winter seems to have been mild, except for the second half of January which was harsh as described by Descartes himself; however, "this remark was probably intended to be as much Descartes' take on the intellectual climate as it was about the weather.")  In 1996 E. Pies, a German scholar, published a book questioning this account, based on a letter by Johann van Wullen, who had been sent by Christina to treat him, something Descartes refused, and more arguments against its veracity have been raised since. Descartes might have been assassinated as he asked for an emetic: wine mixed with tobacco.  As a Catholic in a Protestant nation, he was interred in a graveyard used mainly for orphans in Adolf Fredriks kyrka in Stockholm. His manuscripts came into the possession of Claude Clerselier, Chanut's brother-in-law, and "a devout Catholic who has begun the process of turning Descartes into a saint by cutting, adding and publishing his letters selectively." In 1663, the Pope placed his works on the Index of Prohibited Books. In 1666 his remains were taken to France and buried in the Saint-Etienne-du-Mont. In 1671 Louis XIV prohibited all the lectures in Cartesianism. Although the National Convention in 1792 had planned to transfer his remains to the Pantheon, he was reburied in the Abbey of Saint-Germain-des-Pres in 1819, missing a finger and skull. His skull is on display in the Musee de l'Homme in Paris.  Initially, Descartes arrives at only a single principle: thought exists. Thought cannot be separated from me, therefore, I exist (Discourse on the Method and Principles of Philosophy). Most famously, this is known as cogito ergo sum (English: "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. "The simple meaning of the phrase is that if one is sceptical of existence, that is in and of itself proof that he does exist."  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 unreliable. So Descartes determines that the only indubitable knowledge is that he is a thinking thing. Thinking is what he does, and his power must come from his essence. 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 the person is immediately conscious. He gave reasons for thinking that waking thoughts are distinguishable from dreams, and that one's mind cannot have been "hijacked" by an evil demon placing an illusory external world before one's senses.  And so something that I thought I was seeing with my eyes is in fact grasped solely by 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.

Q: What was one of Descartes' philosophical writings?
A: Descartes arrives at only a single principle: thought exists. Thought cannot be separated from me, therefore, I exist

Q: Did this theory become popular?
A: unknown
How did he arrive at this conclusion?
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;
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;