Descartes began ‘Meditation One’ of his classic Meditation on First Philosophy by saying that throughout his life, he had acquired several opinions and beliefs which he later discovered to be false (17-18). The main goal of Descartes was to find a foundation on which knowledge can be built. That is Descartes wanted to find a certainty or truth which could not be doubted beyond dispute. He agrees with Plato that knowledge requires certainty, but reject Plato idea that physical world is not knowable. What does it means to be certainty? Descartes main problem was how to achieve this certainty. In order to achieve this aim, Descartes adopted a systematic method known as the method of doubt. The method of doubt teaches us to take our beliefs and subject them to doubt. If it is possible to doubt, then we treat them as false, and we need to repeat this process until we are unable to find something to doubt on. The main point is that we treat all our beliefs as false until we find a foundation which is undoubtable, and we used the undoubtable things to build up everything on it. The method of doubt will be elaborated based on: doubting the senses, doubting the physical world (dream), and imagining that there exists an evil genius.