“Sudden illumination,” says Poincaré, is only “a manifest sign of long, unconscious prior work,” work usually done alone. Often when one works at a hard question, nothing good is accomplished at the first attack. Then one takes a rest, longer or shorter, and sits down anew to the work. During the first half-hour, as before, nothing is found, and then all of a sudden the decisive idea presents itself to the mind.
