Perhaps one of the most misunderstood figures of history is Descartes, viewed by many (especially Christians in general) as the fountainhead of Enlightenment philosophy. While Descartes’ method of doubt was indeed something by which subsequent philosophers employed, Descartes is not necessarily the “bad boy” many Christian thinkers make him out to be. Descartes was a devout believer in the Christian God, and his belief in God was the source of his belief that we can know things certainly. What Descartes sought to correct, though, was how he had believed things in the past to be true when they were indeed false. Further, he was reacting against the skepticism of his day in light of the competing truth claims.
Descartes, like many other Enlightenment thinkers, was impressed with how much progress the science of his day was making in helping to further one’s knowledge of the world and how it works. Yet, when one saw the attempts of philosophy, the progress in such knowledge sorely lagged behind. So, Descartes sought to find certainty on something that we all experience (a posteriori), employing the method of doubt to find that certainty. Interestingly, Descartes employed that very method skeptics used to try to destroy one’s certainty of knowledge. Thus, when Descartes doubted his beliefs, he did not necessarily do so because he didn’t believe what he perceived, or because he thinks a demon or God was deceiving him, or even because he wasn’t sure whether he is awake or dreaming – rather, he did so to arrive that which it would be logically impossible and absurd to doubt.
So, when Descartes says in his Meditations that he can doubt that he has a leg, or an arm, or any other bodily extremity, he isn’t saying that he actually doubts this, but that it would not be a logical impossibility that he does not have one of these extremities. Further, in regard to his discussion on dreaming, because dreams can seem so real to the dreamer (such that when they are dreaming, what they experience in the dream can produce effects on the dreamer like those they experience when awake), that one can possibly doubt his being awake as opposed to just dreaming. Again, he is not saying this because he believes this, but as conceding the point to those who are skeptical regarding the certainty of knowledge.
This entire process Descartes goes through is on the assumption that we can easily be deceived by our senses. What we don’t experience through our senses can easily be doubted, and what we can and do experience with our senses can possibly be doubted; thus, Descartes eventually finds that there is not 100% certainty found in our experiences or beliefs. However, and here is what Descartes is famous for, Descartes is able to know with certainty that he is doubting. That is, he may doubt his beliefs, but he cannot doubt the fact that he is doubting, and therefore thinking. Hence, his “I think, therefore I am.” Thus, Descartes knows that he exists because he is thinking.
He also goes on to state that there are basic beliefs – beliefs that are true and do not require evidence or proof to demonstrate their validity – upon which we build our knowledge. The processes of induction and deduction, operating from these basic beliefs, offer one the surest route to knowledge. In order to prove the existence of clear and distinct ideas, Descartes goes on to prove the existence of God and that He does not deceive us to think that false things are true. Immediate ideas that one has must have a cause and this cause must have as much reality as the effects (the ideas). Thus, the idea of an infinite, perfect God must come from outside himself (Descartes); hence, if Descartes has an idea of an infinite, perfect God, then He must by necessity exist, since perfection implies existence.
So, we see that Descartes does not necessarily doubt God or the existence of other things; rather, he uses doubt to arrive at certainty, and for Descartes, he knows for certain his own existence. From this, he entails the reality of basic ideas, and these basic ideas must have a cause outside of himself, and this eventually allows him to demonstrate God’s existence. So, Descartes’ doubt does not end in doubt of all knowledge or truth, but actually leads, for him, to a confidence in knowledge and in the existence of God.
Now, Descartes’ ideas and method do have problems if one were to take his conclusions to their logical end (which is done by Locke, Hume, and eventually Kant), and his method of doubt and his effort to do philosophical thinking in a way modeled after scientific thinking (operating from what we observe/perceive as opposed to beginning with the ideal first and then working from that ideal) serves as a model for other Enlightenment thinkers to eventually discard faith and uphold reason as the judge of all truth. So, it is right to critique Descartes’ method and conclusions (as many of you do), but it must be done so with his motives in mind – Descartes was not out to disprove God or do away with faith in Him.
Here is a PDF of Descartes’ famous Meditations, provided by the University of Connecticut: http://selfpace.uconn.edu/class/percep/DescartesMeditations.pdf