Intellectually, the month of September 1900 was the highest point of my life. I went about saying to myself that now at last I had done something worth doing, and I had the feeling that I must be careful not to be run over in the street before I had written it down. I sent a paper to Peano for his journal, embodying my new ideas.
Source: The Autobiography of Bertrand Russell, v.１, chap. 6: Principia Mathematica, 1967
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