Thought and Reason
Descartes is most famous for his statement “Cogito ergo sum,” translated as “I think; therefore I am.” According to Descartes, the act of thinking is proof of individual existence. Descartes argues that thought and reason are the essence of humanity because while one cannot be sure of any other part of existence, one can always be certain that he has thoughts and reason. For thoughts to exist, there must be a source to do the thinking; therefore if one thinks, one has to exist. To Descartes, humans are also capable of reason, and without it, one would simply not be human.
Descartes believed that it is through the ability to reason that humans gain true knowledge and certainty in science. His assumption that reason is a natural talent gifted to all people led him to write about very complex and philosophical matters in a way that could be understood by all. He even sometimes wrote his works in French instead of Latin (the language used by scholars) so his work could be read by the masses.
Descartes presented arguments as logical trains of thought that anyone would be able to follow. He believed that any problem could be broken up into its simplest parts and that problems could be conveyed as abstract equations. By doing so, one is able to remove the issue of sensory perception (which, according to Descartes, is unreliable) and allow for objective reason to solve the problem.
Since sensory perception was unreliable, the only thing Descartes could truly be sure of was that people are thinking things. Therefore, reason and thought are the essence of all people. And since there is a difference between pure reason and sensory perception, Descartes argues, there must be the existence of the soul.
The Existence of God
Once he was able to establish that man exists solely as a thinking thing, Descartes began to look for other self-evident truths. Descartes concluded that perception and imagination have to exist because they are “modes of consciousness” within the mind, but do not necessarily hold any truths. Therefore, Descartes concludes that the only way to have knowledge of other things is by having knowledge of God.
According to Descartes, since God is perfect, it is impossible for God to deceive someone. Descartes then claims that though he, himself, is imperfect, the fact that he can conceive of the notion of perfection means perfection must exist; and this perfection is God.
The Mind-Body Problem
Descartes was a famous proponent of substance dualism (also referred to as Cartesian dualism), the idea that the mind and body are separate substances.
Descartes believed the rational mind was in control of the body, but that the body could influence the mind to act irrationally, such as when one performs an act of passion. According to Descartes, the mind and body interact with each other at the pineal gland, which he called “the seat of the soul.” According to Descartes, like the soul, the pineal gland is a part of the brain that is unitary (though scientific research now shows that it too is split into two hemispheres), and its location near the ventricles makes it the perfect location to influence the nerves that control the body.
Here is Descartes’s illustration of dualism. Sensory organs pass information to the pineal gland in the brain, and this information is then sent to the spirit.