Saint Isaac the Syrian
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Stillness, as Saint Basil says, is the beginning of the soul's purification. For when the outward members cease from their outward activity and from the distraction caused thereby, then the mind turns away from distractions and wandering thoughts that are outside its realm and abides quietly within itself, and the heart awakens for the searching out of deliberations that are within the soul. And if purity is nothing else save forgetting, an unfree mode of life and departing from its habits, how and when will a man purify his soul who, actively of himself or through others, renews in himself the memory of his former habits ... ? If the heart is defiled every day, when will it be cleansed from defilement? But if he cannot even withstand the action upon him of outward things, how much less will he be able to purify his heart, seeing that he stands in the midst of the camp and every day hears urgent tidings of war ... ? if, however, he should withdraw from this, little by little he will be able to make the first inner turmoils cease. . . . Only when a man enters stillness can his soul distinguish the passions and prudently search out her own wisdom. Then the inner man also awakens for spiritual work and day by day he perceives the hidden wisdom which blossoms forth in his soul. . . . Stillness mortifies the outward senses and resurrects the inward movements, whereas the outward manner of life does the opposite, that is, it resurrects the outward senses and deadens the inward movements.

From the Spiritual World of Isaac the Syrian, Hilarion Alfeyev pp 80-81, [I/37 (173-175) = PR 35 (243-247)]

See also Issac the Syrian On Prayer, Basil the Great On Attentiveness, ot Macarius on Unceasing Prayer