Cogito ergo sum

Descartes most famous contribution to philosophy, I think, therefore I am, is often thought to be a turning point in the history of philosophy, making the certainty of individual consciousness the new starting point for philosophy. The rise of the individual, and the self, as we experience them today

The steps of the argument are quite simple. Descartes first claims that so many former truths have been shown not to stand up to scrutiny, that philosophy needs a new starting point. In his search for this new fulcrum he adopts two methodological devices: not to examine individual claims to knowledge, but rather their basis (sense perception or reason, for example), and to discard as false any statement that isn't evidently true. The latter device allows him to discard anything of which he might have the slightest cause for doubt.

The first victim of his analysis is knowledge based on sense perception. We are often deceived by our senses, so clearly they cannot provide us with the sort of foundational truth we need. Even those sensations that seem so direct and unquestionable to our senses that they couldn't be false, can be questioned when we realize that we often dream that we are feeling them, only to discover later that it wasn't the case. If we cannot distinguish being awake from sleeping, then sense perception can never lead to certain knowledge.