Descartes searched for a method that would enable knowledge to be certain. He looked at the sciences, especially mathematics to based this method. Descartes begins by doubting everything. He asserts, “And thus I realized that once I raze everything to the ground and begin again from the original foundations, if I wanted to establish anything firm and last in the sciences.” He began by doubting everything as a methodology to achieve certainty except for the validity of such a method of doubting everything. This was different from philosophers who preceded him whom accepted common beliefs until they were shown to be unreliable.
Descartes thought the senses could not be trusted. They did not give a certain knowledge, but only a probable knowledge. So, his method would need to be deductive. He concluded that he needed to doubt everything that could be doubted and rebuild only the beliefs that had “valid and considered reasons.” In other words, beliefs must have evidence to support them. He decided that he would put “aside everything that admits of the least doubt. . . I will stay on this course until I know something certain.” He discovered that thought alone was certain. He also had great confidence in his “natural light”, that is, his natural capacity to think clearly and logically. He thought he could not separate thought from himself. Because he thought; he existed. From this foundation, he would build his knowledge base. He thought of himself as something that thinks. He was also a “thing that doubts, understands, affirms, denies, wills, refuses, and also imagines and senses.” He is not his body; he is his mind. He restores our knowledge of the world of extended things (that is, the world of everything beyond his mind, including his body), but only as extended objects that can be known by way of measurement.