The most curious example of this process, and the one which led me subsequently to rely upon it, occurred at the beginning of 1914. I had undertaken to give the Lowell Lectures at Boston, and had chosen as my subject "Our Knowledge of the External World". Throughout 1913 I thought about this topic. In term time in my rooms at Cambridge, in vacations in a quiet inn on the upper reaches of the Thames, I concentrated with such intensity that I sometimes forgot to breath and emerged panting as from a trance. But all to no avail. To every theory that I could think of I could perceive fatal objections. At last, in despair, I went off to Rome for Christmas, hoping that a holiday would revive my flagging energy. I got back to 'Cambridge on the last day of 1913, and although my difficulties were still completely unresolved I arranged, because the remaining time was short, to dictate as best as I could to a stenographer. Next morning, as she came in at the door, I suddenly saw exactly what I had to say, and proceeded to dictate the whole book without a moment's hesitation.