Descartes and Rationalism
René Descartes, 1596-1650 (Latin Renatus Cartesius, hence the term Cartesian)
Descartes’ Project Descartes was a contemporary of Galileo and Kepler. He was born about 50 years after the publication of Copernicus’ De Revolutionibus. Thus he lived right at the beginning of the scientific revolution, as the medieval world view was beginning to collapse. Descartes was a mathematician and physicist, as well as a philosopher. He was the first to offer a system of mechanics that applied both to terrestrial and heavenly bodies. His system was based on a set of laws governing the motions of particles, including various types of collisions. These laws, though unsuccessful, were a precursor of Newton’s laws of motion, and Huygens’ solution to the collision problem. Descartes had the disturbing experience of finding out that everything he learned at school was wrong. From 1604-1612 he was educated at a Jesuit school, where he learned the standard medieval, scholastic, Aristotelian philosophy. In 1619 he had some disturbing dreams, and embarked on his life’s work of rebuilding the whole universe, since the Aristotelian universe was doomed. (Descartes didn’t suffer from lack of ambition!) The problem for Descartes was that he couldn’t merely tinker with the medieval picture, fixing it up here and there, because it was fundamentally wrong. It was rotten to its very foundations. The only way to proceed was to tear it down completely, and start building again from scratch. For an analogy, suppose your computer has been attacked by a virus, so that many important system files have been corrupted. You don’t know which files have been corrupted, so you can’t really trust any of them. The only thing to do is to erase the hard drive completely and re-install everything. How did Descartes “erase his hard drive”? He used what is known as his method of doubt. He tried, as far as was possible, to empty his mind of all beliefs, to suspend judgment about everything. Of course this isn’t easy, as one does not simply choose what to believe. On cannot help believing that things are basically as they seem to be. To help him doubt even things that seem obviously true, Descartes meditated on various possible “sceptical scenarios”. These are situations that cannot be ruled out, i.e. they could be one’s actual situation, yet if they are true then just about all one’s beliefs are false. These are well known. First Descartes considered that, when asleep and dreaming, everything seems just as real and true as when he is awake. So perhaps he is dreaming at this very moment, in which case he may not be sitting in a chair, writing, and so on. To make his doubting even deeper and more radical, Descartes considered the possibility that God is evil (the “evil demon”) and has the aim of deceiving Descartes as much as
possible. All of his sense experience, everything he sees, hears, touches, smells and tastes could be a sophisticated illusion. The demon feeds the fictitious sensory inputs directly to his conscious mind. Even his sense of motion and positions of his limbs could be part of the illusion. (Descartes should have been on the credits of The Matrix! You can also see connections with Berkeley’s later idealism here.) On this scenario, which cannot be ruled out, almost everything he believes is false. An important insight of Descartes, concerning the demon scenario, is that one’s physical body might be an illusion. This extended, geometrical object, with arms, legs, hair, and so on, might not exist. One’s real body might be quite different; perhaps one is really four-legged, feathered, or completely bald? Or perhaps one has no physical body at all! Isn’t it possible that one’s self is a purely thinking “substance” (object) with no geometrical properties like volume and shape? One might be a disembodied soul, receiving fictitious sense experiences from the demon. I cannot be sure, therefore, that I have a physical body. Is there...
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