Descartes employs a method called metaphysical doubt, sometimes also referred to as methodological skepticism: he rejects any ideas that can be doubted, and then reestablishes them in order to acquire a firm foundation for genuine knowledge. Descartes arrives at only a single principle: thought exists. Thought cannot be separated from me, therefore, I exist. We exist as long as we are thinking. We must deny any sensory data that we receive when we look at this approach to our existence. This means we really can't believe anything that we get from our senses is an exact interpretation of reality. There must be something that tells us more then our sensory knowledge. Now we have to see what we know with the absence of our senses. Descartes said the only thing we can be sure of is that we are thinking things. Even if someone told us not to think and clear our mind we are thinking of not thinking. It is not imaginable for someone to think of something at which we are not thinking.
John Locke was an English philosopher. Locke is considered the first British Empiricists but equally important to the social contract theory. His ideas had an massive influence on the development of epistemology and political philosophy. Locke's theory of mind is often cited as the origin of modern conception of identity and the self, figuring prominently in the later works of David Hume and Immanuel Kant. Locke was the first philosopher to define the self trough continuity of consciousness.