Even metals like steel are found in the laboratory tests to suffer from fatigue when over-used. How much more must the delicate elements composing the human body suffer from it? If metals need the rhythm of rest, how much more do we? The living tissue of the flesh shows its wonderful balance in the unconscious action of both diaphragm and heart muscles, where every movement is counterbalanced by a rest. If Nature assigns such an indispensable place to the principle of balance in the human body, it is perfectly logical to believe that she assigns it in the human mind too.
-- Notebooks Category 3: Relax and Retreat > Chapter 3: Relax Body, Breath, Mind > # 30