Descartes" method of radical doubt consists of a process of a doubt. The four methods of radical doubts are: 1) only except clear/distinct ideas; 2) divide your problems into smaller, more manageable parts; 3) order your thoughts from simple to complex; 4) and check your results for errors. .
Descartes uses this method to question his beliefs in the 1st Meditation by accepting the fact that most things we learn in life come from or through our senses. At times, our senses can deceive us, but only with small or far away objects. Our senses rest between primary and secondary qualities. By questioning the distinction of this primary and secondary quality, Descartes must doubt the reliability of his senses. For Descartes, to have doubts about his senses, he must find a reason to doubt both his primary and secondary perceptions. Descartes begins his process of doubting by putting out the optical illusion, which causes the credibility problem with our senses. Optical illusion is indeed the basis the basis of knowledge. If Descartes is not sure if he can build his theory of knowledge on this subject, then he will hold assent on the senses until he is certain that his clear and distinct perceptions are indubitable. Even though Descartes doubt composite things, he cannot doubt the simple and universal parts. Geometry and arithmetic escape the arguments about illusion and dreaming because it "contains something certain and indubitable" (Descartes 466). One can dream that two and three is six, but it is impossible that the answer is different than five because no matter what, it can never be false. The sort of situation Descartes need to create in order subject these "truths" to doubt is the evil demon. .
In the second meditation, Descartes knows for certain the idea of his mind's existence, also known as "cogito ergo sum." Descartes will believe it is true when this deception comes up in his mind because it is the only thing that he is certain.