"I think, therefore I am," said Descartes. But the ordinary man does not stand and wonder how he knows he exists. He assumes his own existence and claims to know physical things as they exist independent of his perception of them. A philosopher may look upon this innocent acceptance as an 'audacious piece of metaphysical theorizing' not at all based on what he calls 'sense data'. The philosopher cannot accept the knowledge of his existence unless it is logically deduced.
Descartes undertook the task of doubting everything that could be legitimately be doubted. The desire to know the true nature of reality, was invoked in Descartes' mind early in his time. Before long he had resolved "no longer to seek any other science than the knowledge of myself or of the great book of the world."
His process was a process of doubt, which doubted the existence of everything.
He wished to adopt the mathematical criterion of certtainity in philosophy. Only a scientific method could free philosophy of dogma and unreason.
Descartes said in his Discourse on Method - "When I consider that some thoughts which we experience when awake may also be experienced when we are asleep, while there is at the time not one of them true, I suppose that all the objects that has ever entered into my mind when awake had in them no more truth than the illusions of my dream. But immediately upon this I observed that whilst I thus wished to think that all was false, it was absolutely necessary that I who thus thought, should be somewhat; and I observed that this truth, I think therefore I am, was so certain and of such evidence, that no ground of doubt no matter how extravagant could be alleged by the skeptics...