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I think, therefore I am. Cogito ergo sum. I reached this by a method of systematic doubt: I doubted everything that could possibly be doubted — my senses, my memories, my reasoning, the existence of the external world, even the possibility that a malicious demon was deceiving me about everything. What remained undoubtable was the fact that I was doubting. And doubting is thinking. And thinking requires a thinker. I could not doubt that I existed, because the very act of doubting proved I did.