In the "First Mediation", Descartes draws out the framework of his project, which is to "achieve [foundations of knowledge which] is solid and lasting". He realised that in order to establish knowledge at a perfect certainty, it is necessary for him to cast doubts on all of his beliefs. The basic strategy of Descartes" method of doubt is to defeat skepticism on its own ground. This is done through the suspension of judgment or the withholding of belief/assent. Therefore, once we are free of all our "preconceived, embedded opinions" we would than be able to start from scratch and rediscover former beliefs based on basic, indubitable propositions. These beliefs would than be the foundations upon which knowledge can be rebuilt.
Descartes goes about using his method of doubt to cast into doubt our beliefs, which we take for granted as being true, through three arguments. This can be seen as an extended exercise in learning to doubt everything that we believe at three distinctive levels. Firstly, Descartes argues that "from time to time I [he] have found that the senses deceive with respect to barely perceivable and distant objects". In other words, the creditability of the senses with respect to any particular judgment of the external world may turn out to be a mistake. This is a perceptual illusion as things are not always what we perceive them to be at first glance (sight) or touch; the true nature of things cannot be judged based on our sensory perceptions. Therefore, it is "prudent never to trust completely those [senses] that have deceived us even once".
Secondly, as reinforcement to doubting the legitimacy of our sensory perceptions, Descartes proceeds with his dream argument. He noted that "there are never any sure signs by means of which one can distinguish wakefulness from sleep". If this is the case than there is a possibility that everything we "perceive" now of the material/external world is nothing but a figment of our vivid imagination.
