“I think, therefore I am” is perhaps the most famous quote in the history of philosophy. Its coiner, René Descartes, is regarded as the father of modern philosophy and one of the first thinkers to blend the disciplines of philosophy, science and mathematics.

Descartes was born in 1596 in the small town of La Haye in Brittany, France. His father was a high court judge and Descartes enjoyed a cultured and pampered childhood. At the age of 10, Descartes was sent to a Jesuit school in the province of Anjou. Under his religious masters he studied classics, history and philosophy. He was accepted into the University of Poitiers and studied law, graduating in 1616.
While Descartes was a Catholic, he joined the Protestant Dutch Army and spent the next three years as a soldier. While campaigning for the Duke of Nassau, Descartes met a fellow soldier Issac Beeckman who introduced him to mathematics.
In 1619 Descartes returned to France and took up residence in Paris. For the next eight years Descartes studied and practiced law. In 1628 Descartes left France and settled in Holland. The next 20 years saw him devote himself to philosophy and thought. He wrote extensively although chose not to publish at first.
He rejected scholastic philosophy and the sort of skepticism described by fellow Frenchman Montaigne. Descartes wrote of a theory to apply mathematical methods to the achievement of certainty in human knowledge.
In 1628, Descartes wrote Regulae ad directionem ingenii (Rules for the Direction of the Mind) which expounded his ideas and explained his principles and method. In 1634 Descartes completed Le Monde (The World), which supported Galileo’s astronomical theories but he refused to release it publically after the Inquisition attacked and investigated Galileo. 
It wasn’t until 1637 that Descartes finally published his thoughts to a wider audience. Descartes wrote about analytical geometry, dioptrics, which was a substantial explanation of optics and meteorology. The preface to his theories was called  Discours de la méthode pour bien conduire sa Raison et chercher la Vérité dans les Sciences   (Discourse on Method) This preface discussed the problems with traditional education, proposed new methods of instruction and explained in detail his method and principles of his unified attempt at human understanding.
Four years later, Descartes offered (in Latin) a more formal explanation of his main ideas in Meditationes de Prima Philosophia (Meditations on First Philosophy). In this book Descartes argued skepticism is overcome by the certainty of one's own existence. Descartes also proposed a specific dualism, in which the mind and body are wholly distinct, even though they interact. Finally, Descartes combined his views in Principia Philosophiae (Principles of Philosophy) published in 1644.
Descartes's became famous throughout Europe and he began a series of correspondence with may of Europe’s leading thinkers. 
In 1649, Descartes was invited by the Queen Christina of Sweden to teach her philosophy establish an institute for sciences in Stockholm. Descartes arrived in late 1649, but became ill with pneumonia and died in Stockholm in February, 1650.

Discourse on Method

Discourse on Method was written in French and not in the more conventional Latin, and was written for a wider audience. Divided into six parts, Descartes plays the role of Socrates in search of truth and wisdom. In parts one and two, Descartes details his early philosophical doubts, and his discovery of his "method". He explains four rules for reforming ways of thinking: accept nothing that is not clear and distinct; second, divide difficult subjects into many small parts; third, start with the simplest problems; fourth, be comprehensive. Descartes then explains his systems of morality and metaphysics. The essay concludes with observations on astronomy, physiology, meteorology and optics.