René Descartes wrote in his seminal book, Discourse on Method (1637), “There is a great difference between the mind and body.” It is here that this luminary figure from the Age of Enlightenment introduces his theory of dualism. Not only was he a great mathematician, but René also contributed several philosophical notions to his name: the Demon Hypothesis, skepticism, and the famous Cogito Ergo Sum.
During childhood, Descartes was afflicted by tuberculosis, which meant that he was constantly bedridden. It was from these days in bed at a Jesuit college that René claimed he had a series of dreams that lead to an epiphany. Awakening from this experience, a young Descartes couldn’t discern whether he was truly awake or if he was still dreaming. René became a skeptic, doubting everything in the material world. He states in the Demon Hypothesis that an evil embodiment of God has disillusioned us into thinking we are in control. But alas, there must be some proof of existence, René thought. And so, René famously said, “Cogito Ergo Sum,” or I think therefore I am. He figured that, while in this eternal sleep, there must be a dreamer who is dreaming this dream. Thus, he arrived at the conclusion that this state of thinking proved his spirit’s existence. Dualism is the idea that the mind and body are separate, and many of Descartes works are based on this conception. So if the body and mind are two different, disconnected things, then how do they communicate with one another? Descartes argued that the pineal gland in the brain acted as a liaison between the two. What you perceive physically is transferred to the central gland, and vice versa with sensory perception. Although René Descartes may not be scientifically correct, his theories have provided wonderful insights in math, philosophy, and psychology.
For further reading: 1001 Ideas That Have Changed The Way We Think by Robert Arp (2013)
501 Things You Should Have Learned About Philosophy by Alison Rattle
Essentials of Philosophy by James Mannion (2006)
The Psychology Book by DK (2012)