The history of modern philosophy is normally divided into three sections, empiricists, rationalists, and Kant, with the latter offering a synthesis of the other two. Roger Scruton explains the distinction:
Very roughly, empiricists believe that all knowledge is derived from, and justified by, sensory experience; rationalists hold that knowledge is obtained only through rational thought; while Kant argues that knowledge involves a synthesis, in which the faculties of sensation and thought come together (Scruton, 442).
Among the rationalists are Descartes, Spinoza, and Liebniz, and among the empiricists are Bacon, Locke, Berkeley, and Hume.
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:
I observed that there is nothing at all in the proposition "I am speaking, therefore I exist" to assure me that I am speaking the truth, except that I see very clearly that in order to think it is necessary to exist. so I decided that I could take it as a general rule that the things we conceive very clearly and very distinctly are true; only there is some difficulty in recognizing which are the things that we distinctly conceive (Cottingham, Stoothihf, and Murdoch 127).
Descartes notes a number of reasons why we have to doubt our senses. For one thing, the senses sometimes deceive us with regard to minute objects or objects that are at a great distance from us. Our senses have limitations when it comes to things that are too small or too far away. There are other things which are self-evident and not to be doubted, such as ...