The most important thing to remember about Descartes is that he rejected experience as a valid basis of knowledge. By experience, I mean sense data - sight, sound, etc. In other words, everything that science relies on; for physical experiments that science relies on are nothing more than a formal form of observation. His aim, as most clearly set out in the Meditations and The Discourse on Method, was to establish an absolutely true system that no one could doubt. Descartes aimed for absolute certainty. His inspiration was probably Euclid's mathematical system which was at the time felt to be flawless. Euclid proceeded to establish mathematical truths by starting with axioms that were universally accepted, e.g. a triangle is a shape which has 3 sides. From these self-evident truths he proceeded to reason out new truths, such as that the angles of a triangle all equal 180 degrees; theoretically this meant that there could be no error in the conclusions, provided the reasoning was solid. Descartes wanted to use this method to form a system of epistemology, metaphysics and physics.

Descartes considered that sense data clearly deceived us sometimes. For example if we look at objects under the surface of the water, they usually seem distorted, and have no solid shape, even though in reality they are perfectly solid; or how from a distance things look different than they actually are. Descartes argued that we could not achieve certainty by trusting senses which we knew had deceived us in the past, for we could never say for sure when they were deceiving us again. He also noted that we often have false sense experiences whilst dreaming, and usually don't know when we are dreaming. He also argued that God, or a malicious demon, could be deceiving us and feeding us false sense data (the film 'The Matrix' provides a good example of this idea). For these reasons he rejected sensory perception as being able to provide knowledge which was absolutely certain.

He turned instead to pure reason (logical deduction), arguing that if he could establish just one certain truth purely with reason, he could use that to establish a whole system, building on that truth with a series of deductions, as Euclid had done with mathematics. The contrast between the two different methods is certainty. In the scientific method of observation, we only know things act in a certain way through experience. We cannot deduce from a purely mental consideration of magnesium and of fire that magnesium will burn, we can only learnt that by putting magnesium in a fire; and that experience is obtained through our fallible sense perception. Whereas, in Descartes' deductive method, we can deduce truths merely from considering the objects in question; for example, we do not need experience to tell us that all points on a perfect circle's edge will be equally far from the center, the concept of a circle contains that information.

The certain truth Descartes hoped to base his system on was the 'Cogito'. This is his famous assertion that "I think, therefore I am". Descartes' point here was that no matter how you consider the matter, you must exist. Even if everything you believe is wrong, and a demon is deceiving you on everything, there must be a you to deceive. To claim a demon could be tricking you into thinking you exist still says that you are something - how could a demon trick a nothing? You have thoughts, you think, you consider, something must be doing these actions, and therefore you must exist. The cogito makes no claims as to what constitutes a person, but it proves that a person is at least an existing something.

Descartes then argues that the cogito is a 'clear and distinct perception'. That is, it is an idea which is clear, meaning that either you can intuitively grasp it and agree with it once you've been told it, or once it's meaning has been fully explained to you, you intuitively understand that it is true; and distinct meaning you understand that it is one particular truth, and not another. A good example of a clear and distinct perception is 'a thing cannot exist and not exist at the same time'. This is intuitively true, and we understand that this truth is not 'a triangle has 3 sides'. Descartes argued that since the cogito was clear and distinct and undeniably true, we should reason out clear and distinct perceptions for everything, and these would be the truths about which we could be certain.

Here Descartes finds that his conception of an omnipotent God is a problem, for an all-powerful God could even deceive us into thinking clear and distinct perceptions are true when they are not. Descartes attempts to prove the existence of God, and then argues that since deception arises from imperfection in the deceiver, and God is perfect, He would not deceive us, and thus circumvents this problem, and goes on to deduce many more truths.

Objections

There are many objections to Descartes' project. Most philosophers now believe that sensory perception is our only method of obtaining new information, so we have to rely on it, flawed or not (and most agree it is flawed). Pure reason alone cannot lead us to new truths. Descartes' physics was based on pure reason and is now known to be deeply flawed and mostly incorrect, which backs up these claims (for example Descartes deduced that a vacuum could not exist in nature, and we now know that he was incorrect and vacuums do indeed exist). Also, although Descartes says he will base his system on the cogito, in fact this is not what he does at all. He uses it as a model, he does not derive any truths from it, instead he derives truths from his ideas on God. Unfortunately for Descartes his ideas on God are deeply flawed, as I will go into in a separate (forthingcoming) section.