Born in La Haye, France, René Descartes came from a wealthy family but was in poor health for most of his life. His mother died with he was very young; his father (a lawyer) traveled a lot) He was raised by his grandmother, together with an older brother and sister.
Educated at the College of La Fleche (a Jesuit school), Descartes believed that God created the universe, set it in motion, and left it alone. He held that since God was not involved in the day to day operations of the universe, it is possible to study the universe and its laws without making theological statements.
Descartes maintained that animals were basically machines but man has a soul. The body operated like the fountain at St. Germain (outside of Pairs) with its mechanical statues. As people walked on the stones near the fountain, they triggered hidden plates under the stones that were attached with strings and levels to valves that released water through tubes that make the statues move. Similarly, the human body has senses that are connected by thin strings to cavities. Pulling on the strings (stimulating a sense) caused the cavities (ventricles) to release gases to flow to the muscles and move the body. The strings are inside of hollow tubes (nerves) and the gasses were thought to be distilled from blood and were called animal spirits (in the same way that drinks which contain distilled alcohol are called distilled spirits). Just as God is a spirit and can travel anywhere instantaneously, animal spirits move animals and travel instantaneously through the hollow nerve tubes.
The eyes were thought to be connected to directly to the pineal gland. Descartes was a dualist (both body and soul exist) and reasoned that the soul and the mind had to meet somewhere. He proposed that the pineal gland (since it has no duplicates) was where the meeting occurred. The eyes send gasses through the nerves to the pinal gland and make an impression of the scene; that’s why the eyes are the “mirror of the soul.”
After Descartes earned his law degree from the University of Poitiers in 1616,. he set off to see the world. He traveled through Brittany, Switzerland and Italy, and served a volunteer solider for Maurice of Nassau, and for the Duke of Bavaria. On November 10, 1619, Descartes had an encounter with God. In a dream, he had a vision that God was going to reveal all knowledge to him. Descartes’ revelation was that all of the different areas of knowledge can be unified by a single method of reasoning. The method that came to him began with a search for absolutes.
To build a unified body of knowledge, one must start with the smallest parts. These building blocks must be the clear, un-doubtable and simple (not composed of other ideas). From this base of absolute knowledge all other knowledge can be deduced. Descartes searched for the smallest parts of knowledge by systematically doubting everything. In 1629, Descartes, living in Holland, came the end of his search. He had doubted everything, even whether he existed. The only idea he couldn’t doubt was that he was thinking. Consequently, his existence could be assured; “I think, therefore I am.”
In contrast to scholasticism (demonstrating the logical validity of truths), Descartes believed his approach allowed the uncovering of truth. Syllogisms were only useful after a discovery has been made (revealed by authority). Descartes rationalism rejected that reliance on authority (since all authority is flawed). His approach, called the Cartesian method, used both intuition and deduction. Intuition is pure reason (understanding an idea directly, not through the senses); deduction uses memory and draws conclusions from a continuous chain of thought (A to B, B to C, etc.).
Although a rationalist (noting the faultiness of sensory perceptions), Descartes accepted both innate ideas and empirical observations. According to Descartes, knowledge of God is innate, supported by reason (perfect ideas can’t come from something imperfect, so they must come from God). Similarly, even though perceptions can be faulty, experiments in physics, biology and physiology provide confirmation of an ideas validity. Ideas must be tested in reality. Indeed, Descartes believed that all of science is a tool for bettering human life; ultimately, theory must be turned into practical applications (e.g., ethics, medicine, mechanics).
Descartes’ discovery of analytic geometry had both theoretical and practical consequences. While lying in bed, he was idly watching a fly in his room. Descartes noticed that he could describe the fly’s location with only 3 numbers (one for each dimension). From a theoretical perspective, this coordinate system integrates geometry and algebra. In practical terms, it allows a mathematical description of planets in their orbits, arrows in flight and people in their environment.
For more on this topic, here’s a video on the Mind-Body Problem.