Descartes (pronounced “day-cart”) was a French philosopher and mathematician. He is considered the father of the modern scientific method and the father of modern, western philosophy. Descartes wanted to recreate philosophy from the ground up (essentially throw out all of the old mistaken notions and rethink these problems given our new understanding of mathematics and physics). The story is that on November 10, 1618 he had a dream which he interpreted as a sign that he should found a unified science of nature based upon mathematics. This is important because TRUTH in mathematics is absolute, not based on opinion or probability. He believed that all sciences could meet this standard, including human nature.
So Descartes, while not a sceptic per se, took an extreme skeptical position by asking, “What is impossible to doubt, even when trying to believe that everything is false?” His answer was:
I noticed that while I was trying to think everything was false, it must needs be that I, who was thinking this, was something. And observing this truth, I am thinking, therefore I exist was so solid and secure that the most extravagant suppositions of the sceptics could not overthrow it, I judged that I need not scruple to accept it as the first principle that I was seeking. -Discourse (Part IV)
I think, therefore I am (Cogito ergo sum). This is the one thing that Descartes believes cannot be doubted. This also leads to his assumption that Free Will is self-evident. If one is capable of doubting the existence of things learned through experience even when some of these may be true, then it is obvious that we have the freedom to disbelieve, thus free will.
Cartesian Dualism - The problem for Descartes is two-fold. First, if humans are free, then that which is free is not subject to the laws of physics (extension). However, all physical things are subject to the laws of physics (by definition). Therefore, the mind (the source of free will) must not be physical in nature. It must be distinct from the body. (Human Beings are composed of two parts - Mind (Soul) and Body. This division of humans into two parts is Cartesian Dualism. The presence of a mind allows human beings to transcend their physical bodies and be free.
However, the mind is affected by the body. The soul is not a principle of life (think of a watch or moving statues at Versailles). The soul moves and feels in the body directly. The pineal gland allows the mind and body to interact. The mind controls the direction of actions, but not the speed or power. Descartes believed that the mind is also divided into two aspects, perception (sentience - operation of understanding) and volition (judgement and motivation- operation of volition). Perception is connected with the physical world and volition is completely free of it.
For some reason, Descartes would not split these two aspects of the mind. He uses the capacity for language as evidence of the presence of a mind ratiocination). He believed only humans have language. Therefore, only humans have a mind. Since the mind includes sentience, Descartes is led to conclude that all animals lack sentience. Thus, even though they cry out when injured, they do not feel (sense) pain.
Descartes’ dualism creates a moderate position between religious doctrine and scientific progress. Descartes is a religious man of science. He is also mindful of the condemnation of Galileo when he taught the Copernican system of cosmology. Descartes does not want to suffer a similar fate. By splitting the mind and the body, he can then claim that scientific progress need not undermine church authority. Science is primarily concerned with the body. Religion is concerned with the soul (mind). Since the soul is the more important element he thought the religious leaders could allow science to continue without threat. Descartes wrote in Principles of Philosophy (1644) that he hoped that his views on cosmology could “be used in Christian teaching without contradicting the text of Aristotle.” But, this didn’t happen.
But Descartes did agree with Aristotle that the perfection of human beings is free will (which comes directly from the ability to think - perception and volition.
I leave you with a quote from his Second Meditations.
There is no doubt that I exist, if he is deceiving me; let him deceive me as much as he likes, he can never bring it about that I am nothing, so long as I think I am something.
Descartes is considered to be to philosophy and science, what Martin Luther was to religion. He wrote all of his works in French instead of Latin so that people outside of academia could read them.