The agitations of the emotional and passional nature prevent a man from attaining this mental quiet. If he has not built up its power by practice, or got it by grace, they cause him to lose it. These include both the pleasant and the unpleasant feelings, the desires and the cravings as well as the sorrows and anxieties and lusts, excessive pleasure and excessive pain. The art of mental quiet can be pushed to a deep inner stillness and by practice can be inwardly maintained in the midst of outward activity. This is why the value placed on keeping calm is very high in both yoga and philosophy.
-- Notebooks Category 24: The Peace within You > Chapter 3: Practise Detachment > # 309