Epistemology is a branch of philosophy which deals with the nature of knowledge. What does that mean? well epistemology basically asks questions about knowledge. What is it? Is it possible to acquire it? What types of things can I know? How do I know what I know? and so on. These questions might sound trivial, but deep thinking is required if we are to soundly answer any of them.
Knowledge is in its most basic form justified true belief, and like everyone else Descartes was just your ordinary philosopher/mathematician/genius trying to show that he was justified in believing what he did. Descartes was not satisfied with dogma, or common beliefs; instead, Descartes wanted 100% certainty that what he believed was justified. To do this he developed a systematic method by which he could doubt everything. If there were beliefs which could not be doubted, then they could serve as a foundation for more beliefs until everything that was true, was justified. The Cartesian Method of Doubt relies heavily on the epistemic closure principle which states that I know that X happens, and I know that X happening entails Y happening, then I know that Y happens. By using this principle Descartes makes what was a pretty profound argument for skepticism. In his first meditation descartes states that: P1. If I know that the external world exists than I know I am not dreaming. P2. I do not know that I am not dreaming. C. I do not know that the external world exists. (side note: Descartes is not arguing that the external world does not exist, he is merely pointing out that we can doubt its existence.) Now you can substitute many things for his example of dreaming. In fact, Descartes himself gives similar arguments using different examples (the Evil Demon, Distorted perception ect.). Basically to understand his argument all you need to do is understand that since we do not know that we are not dreaming, how can we know the world around us is real? Life could be illusory and we would never know it. Since dreams distort our perceptions and senses why couldn’t our perception of the external world be a false reality? Using the example of the evil demon: maybe our minds are simply being controlled by a malevolent demon who cruelly decided to give us a false sense of reality. Since we do not know that this is not the case, we do not know that our perception of the external world falls in line with reality.
After he figured out things we cannot know (according to his view of justified true beliefs). Descartes continued his quest for justified belief in his second meditation by finding that in which we can know, and cannot doubt. This is where Descartes uses the famous phrase “cogito ergo sum” roughly translated “I think, therefore I exist” to demonstrate his main point. Descartes cannot rationally doubt his own existence. For if there is doubting, then their is one doing this doubting. He does not try prove that he exists, only that it is irrational to doubt his existence. If he does not exist, than their is nothing doing the doubting ect.
Now that he had this foundational belief which could not be rationally doubted, Descartes set out to build on this foundation. Most philosophers agree that he was unsuccessful in his later meditations, trying in one to prove that God exists, and that this God (being omni-benevolent) would not give us a false sense of reality. Regardless of the soundness of his later works Descartes’ work serves as a great foundation for skepticism, and epistomology in general.
Posted by Philip Davis 
