What are the four main principles of DesCartes's method?

Descartes created a four step philosophical method, with the first step stating that you should doubt everything that is not true beyond a shadow of a doubt. Then, once you have questions, you have to divide them into sections to deal with one at a time, starting with the simplest and progressing to the more difficult. Finally, you need to continuously go over what you have learned to keep it fresh in your mind as you progress.