Descartes opens his Meditations indicating his desire to have only true beliefs. One way to accomplish this is to doubt everything he has learned that might be subject to error. He does not intend to doubt the truth of every specific idea that comes into is head, but, instead to undermine the foundation of his views.
             To determine if our beliefs are justified, we have to be able to trace them back to a statement, belief, or proposition that cannot be doubted. Descartes saw that Mathematics held the only true and believable facts. If achieved, such a proposition could place the firm foundation on which all subsequent beliefs could be grounded; it would guarantee that all subsequent claims based on it would be true.
             In his writings, he ultimately hopes to achieve absolute certainty about the nature of everything including God, the physical world, and himself. It is only with clear and distinct knowledge of such things that he can then begin to understand his true reality. In order to acquire absolute certainty, Descartes must first lay a complete foundation on which to build up his knowledge. The technique he uses to lay this base is doubt. If any belief can be doubted, it is not certain, therefore making unusable as a foundation.
             Descartes starts by looking at our usual sources for truth. Authority, which is churches, parents, and schools, he says, are not reliable sources because time shows we all die, and that we are eventually proved wrong, much in the same way the accepted truths of Science have changed dramatically over the course of history. Also, he considers the generally accepted view that our senses dependably report the absolute nature of reality. Like authority though, Descartes discards the senses as a source of truth because of the belief that based on the senses there is no definitive way of proving that you are dreaming or that you are awak
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