For anyone to be able to hold the mind utterly free of all thoughts and absolutely cleared of all images is an uncommon achievement. Even when successful, the effort seldom lasts longer than a few minutes. But after that short space of time, those particular thoughts and those particular images which first rise up are important, valuable, or suggestive. They should be carefully noted or remembered.
-- Notebooks Category 23: Advanced Contemplation > Chapter 7: Contemplative Stillness > # 326