... doubting itself, and concluded that he could not doubt that he was doubting. He reasoned: "If I doubt that I am doubting, I am still doubting." The very operation of doubting confirmed its own existence. It was self-validating. Therefore, Descartes could be certain of at least one thing: that while he doubted the existence of everything else, he could be certain that he was doubting. And since doubting requires thinking, he could say: "I think" (in Latin,cogito). But to say "'I' think" means "I" must exist. And so we arrive at perhaps the most memorable slogan in the history of Western philosophy: 3. 'I think, therefore I am.' The famous Cartesian cogito, ergo sum. This was Descartes' "Archimedian fulcrum," the single fixed point of certainty by which he believed he could move out of his world of doubt. This was his "eureka," his "open sesame" to a philosophy and science of certitude. ...

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