The problem of knowledge is a key one in philosophy, asking as it does whether there is anything we can really know and whether what we know can be said to be objectively true. Theories of knowledge come under the heading of epistemology. We perceive the world through our sense, but our senses can be deceived. The degree of deception involved also varies according to different views of the world. Some see this deception as absolute and deny that there can be any knowledge at all through the senses. Others admit knowledge acquired through the senses while recognizing that there are limitations. RenT Descartes was a rationalist, and his thinking was governed by his knowledge of and dedication to mathematics, which he believed could clear up the confusions and uncertainties of philosophy. In this regard, Descartes wished to attain certainty with reference to the external, physical objects in the world around us. Mathematics, he believed, could help philosophy achieve absolute certainty so that philosophy could then reach final and certain truth. He began with the now well-known proposition, "I think, therefore I am," and from this he built a philosophical structure striving for the aforementioned mathematical certainty.
Descartes began with his method of doubt, undertaken because he had reached an age where he now believed that he would be able to remove all of his earlier beliefs and begin with a clean slate, as it were. He says that everything he knew or thought he knew in the past was based on sense perception, and the senses can lie. Because of this, Descartes has to begin from a position of doubt and must prove everything to himself through the application of reason. Descartes has reached the point, as he notes, when he can free his mind from all cares and secure for himself "an assured leisure in peaceful solitude" (598) enabling him to apply his reason to these issues, though it may not be possible for him to show...