Cogito ergo sum

Cogito ergo sum is the Latin translation of Descartes' most famous contribution to philosophy, I think, therefore I am. This idea is often thought to be a turning point in the history of philosophy, making the certainty of individual consciousness its new starting point.

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.

The truths of mathematics would seem a more probable case for certain truths, especially for a mathematician like Descartes, but he introduces the possibility that these truths could have been introduced into our minds by God who wished to deceive us by making us think they are true. He soon backs away from this idea, claiming God could not possibly wish this evil on us, and changes him for an evil demon powerful enough to achieve the same result. Given this new hypothesis not even the truths of mathematics can be saved.

At this point Descartes realizes that while he can subject everything to doubt, the only thing that he cannot dismiss is the very fact that he is doubting. No form of thought can be skeptical of its own existence without serious contradiction, so thinking in its various mental guises, is something of which he can be completely certain. Thinking is proof of my existence!