Developing a quiet mind

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. It is probable in your case that this will come as soon as these descents of peace, intensity or Ananda get strong enough to occupy the whole system.

Ref: Letters on Yoga – IV