The Limit of Deception
In his search for certainty, René Descartes subjected all his beliefs to the hypothesis of an all-powerful, malicious demon. This demon could make him believe he had a body when he did not, and make him perceive a physical world that was entirely illusory. However, in the Second Meditation, Descartes hits upon a logical limit to this deception. He realizes that no matter how hard the demon tries to deceive him, the demon can never bring it about that Descartes is nothing, so long as Descartes is thinking.
The Logic of the Cogito
The core of the argument is elegant in its simplicity. Deception is an activity that requires a target. If the demon is actively deceiving Descartes, then there must be a "Descartes" there to be deceived. The act of doubting, the act of being misled, and the act of thinking are all conscious processes. Therefore, the very existence of these thoughts—even if they are entirely false or manipulated—proves the existence of the thinking subject. This is the essence of the famous phrase, Cogito, ergo sum ("I think, therefore I am").
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By establishing the Cogito, Descartes successfully defeats the radical skepticism of the Evil Demon. The demon's power is no longer absolute. There is at least one truth that is completely immune to deception: the immediate, subjective existence of the conscious mind. From this single, indubitable point of certainty, Descartes begins the process of rebuilding his philosophy, eventually attempting to prove the existence of a benevolent God and the reality of the external world.