Some years ago I read somewhere a memoir by Bertrand Russell. He wrote about the difficulties he encountered in writing Principia Mathematica (or trying to solve the paradox that bears his name). He wrote something like, "In the morning I sat down at my desk in front of a white sheet of paper, and in the evening I realized that I was still sitting in front of a sheet of paper without a line written on it."
Please help me find the source where I could see this quote.