I was trying hard to solve the contradictions mentioned above. Every morning I would sit down before a blank sheet of paper. Throughout the day, with a brief interval for lunch, I would stare at the blank sheet. Often when evening came it was still empty. ... the two summers of 1903 and 1904 remain in my mind as a period of complete intellectual deadlock. It was clear to me that I could not get on without solving the contradictions, and I was determined that no difficulty should turn me aside from the completion of Principia Mathematica, but it seemed quite likely that the whole of the rest of my life might be consumed in looking at that blank sheet of paper. .
Source:The Autobiography of Bertrand Russell, v.１, chap. 6: Principia Mathematica, 1967
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