To get rid of the random thoughts of the surface physical mind is not easy. It is sometimes done by a sudden miracle, as in my own case, but that is rare. Some get it done by a slow process of concentration, but that may take a very long time. It is easier to have a quiet mind with things that come in passing on the surface, as people pass in the street, and one is free to attend to them or not—that is to say, there develops a sort of double mind, one inner silent and concentrated when it pleases to be so, a quiet witness when it chooses to see thoughts and things,—the other meant for surface dynamism.
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There is always a difficulty in keeping the physical mind within or silent, because it has been its nature to occupy itself with outward things and it finds a difficulty in accustoming itself to a contrary movement. You must not be depressed by that, but persist in the aspiration and will till it is done. The Mother’s Force will be there to bring it about as soon as possible.
Sadhana on the Level of Mind, Letters on Yoga – IV, CWSA, Sri Aurobindo.