
Often … [ideas] arrived suddenly, without any effort on my part, like an inspiration.… They never came to a fatigued brain and never at the writing desk. It was always necessary, first of all, that I should have turned my problem over on all sides to such an extent that I had all its angles and complexities “in my head.” … Then … there must come an hour of complete physical freshness and quiet well-being, before the good ideas arrived. Often they were there in the morning when I first awoke.… But they liked especially to make their appearance while I was taking an easy walk over wooded hills in sunny weather.