One evening René Descartes was enjoying the wine at his favorite local bistro when the publican asked if he would like some more wine. René replied, "I think not," and promptly disappeared in a puff of logic.
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But Descarte returned as quickly as he had left, saying, "I have found that even as I said, 'I think not.' I found that in doing so, I was reaffirming and could not deny that I was indeed thinking about wine. Therefore, I most certainly do want that glass of wine.
In short one can doubt one’s own existence but one can’t doubt that one is doubting one’s own doubtful existence! It’s the only one self evident truth that any sentient person can’t ever deny! The “I am” in a person cannot deny his own “I am’ness”.(disclaimer: not using “I am” phrase in the sense that God used it out of the Burning Bush)