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In Meditation 1 we examined how Descartes took down his old house. In tearing down the old house, Descartes has considered that his senses deceive him about things at a distance such as the shape of towers, and the size of stars. This, however, does not call into doubt things which he experiences by these same senses close at hand. He then considered that he might be dreaming and that he cannot distinguish between being awake and being asleep. This hypothesis, however, leaves him assured of the truths of mathematics. Finally he summons up the malin genie, the evil demon, with powers as great as those of God. But while God may be supposed to be benevolent, the demon's whole purpose is to deceive Descartes. This is a more powerful skeptical hypothesis than any imagined by ancient or Renaissance skeptics. Even the truths of mathematics can now be doubted.

Descartes tells us that the demon may try to deceive him into thinking that he does not exist when in fact he does. He discovers that this is not possible, for if he is being deceived, he must exist. So, Descartes has found a truth which he cannot doubt even in the face of the most powerful skeptical hypothesis one can imagine. This truth is a particular case of another such truth: "Cogito ergo sum" or "I think therefore I am."

You many recall from studying Renaissance skepticism that universal skepticism makes the claim that nothing can be known. Academic skeptics claim to know just one thing -- that nothing can be known. Pyrrhonian skeptics claim that they don't even know that. In discovering the cogito Descartes has defeated both of these forms of universal skepticism. Thus, the discovery of this one truth, known beyond doubt, has important philosophical consequences. This is the result of the first application of the methods of doubt and analysis.

Having discovered that he exists, Descartes now asks himself what he is. He does not want to go wrong about this. So we get the second application of the method of doubt and analysis. Descartes goes back and asks himself what he thought he was before he began meditating, and proceeds to eliminate answers along much the same path which he took in reaching the cogito -- first considering answers derived from the senses, then from the imagination and finally from the understanding.

The results of this second application of the method of doubt are that Descartes discovers that when he applies the same set of skeptical hypotheses to himself, the only thing which remains is thought. So, he concludes that he is essentially a thinking thing.

Notice that Descartes has now discovered two truths about the self. The three great categories of Christian metaphysics are God, the self and the body. Descartes is now going to turn to the body. In Meditation 3 he will discover truths about God. Just as the first two truths were discovered by the methods of doubt and analysis, the third application of the method reveals truths about material things.

Essential properties are properties which a thing must have in order to exist. If it loses those properties it ceases to be that individual thing. Life is an essential property of cats. If a cat loses its life it ceases to be that individual cat. Accidental properties are properties which a thing can gain or lose while continuing to exist. If I lose my summer tan, I continue to exist despite the loss. Descartes uses this distinction to show that the essence of material things is known by the mind and not the senses. He takes a piece of wax and shows that all of its sensible qualities can change, but it still remains the same piece of wax. He concludes that the essence of the wax is known by the mind and not by the senses or the imagination.


The first application of the Methods of doubt and analysis gives Descartes the cogito. Then he goes back to the beginning in asking himself what he is. This is the second application of the methods. Finally we shall find that there is a third application which will reveal the essence of material bodies.

Towards the beginning of Meditation 3 Descartes decides to reflect on the indubitable truths he has already discovered -- that he exists, that he is a thinking thing, and that bodies are in essence extended, flexible and changeable -- in order to determine what the criterion of certainty is. He decides that in every case, what these truths have in common is that they are composed of clear and distinct ideas. So here is the criterion of certainty for Descartes "...whatever I clearly and distinctly grasp is true."

Descartes decides that he must next prove that God exists. We will not follow him into the labyrinth of his proof for the existence of God, but it is important to understand why he needs to prove that God exists. Proving that God exists is the only way in which Descartes can effectively remove the doubts caused by the evil demon hypothesis. For if God exists, then there can be no evil demon of the kind which he has imagined. If God were to allow such a demon to exist, God would himself become a deceiver. And this is not possible.


In this unit you will try to understand how Descartes defeats universal skepticism. By showing that a whole range of truths are known for certain and are self-evident, Descartes lays the foundation for asserting that many other truths known by the imagination and the senses can also be known to be true. Your activity in this unit will be to write an essay in which you explicate one of the central terms of Descartes account of knowledge -- the natural light.