Sri Aurobindo
Bases of Yoga
Bases of Yoga
To reject doubts means control of one’s thoughts — very certainly so. But the control of one’s thoughts is as necessary as the control of one’s vital desires and passions or the control of the movements of one’s body — for the Yoga, and not for the Yoga only. One cannot be a fully developed mental being even, if one has not a control of the thoughts, is not their observer, judge, master,— the mental Purusha, manomaya puruṣa, sākṣī, anumantā, īśvara. It is no more proper for the mental being to be the tennis-ball of unruly and uncontrollable thoughts than to be a rudderless ship in the storm of the desires and passions or a slave of either the inertia or the impulses 
of the body. I know it is more difficult because man being primarily a creature of mental Prakriti identifies himself with the movements of his mind and cannot at once dissociate himself and stand free from the swirl and eddies of the mind whirlpool. It is comparatively easy for him to put a control on his body, at least on a certain part of its movements; it is less easy but still very possible after a struggle to put a mental control on his vital impulsions and desires; but to sit like the Tantric Yogi on the river, above the whirlpool of his thoughts, is less facile. Nevertheless, it can be done; all developed mental men, those who get beyond the average, have in one way or other or at least at certain times and for certain purposes to separate the two parts of the mind, the active part which is a factory of thoughts and the quiet masterful part which is at once a Witness and a Will, observing them, judging, rejecting, eliminating, accepting, ordering corrections and changes, the Master in the House of Mind, capable of self-empire, sāmrājya.
The Yogi goes still farther; he is not only a master there, but even while in mind in a way, he gets out of it as it were, and stands above or quite back from it and free. For him the image of the factory of thoughts is no longer quite valid; for he sees that thoughts come from outside, from the universal Mind or universal Nature, sometimes formed and distinct, sometimes unformed and then they are given shape somewhere in us. 
The principal business of our mind is either a response of acceptance or a refusal to these thought-waves (as also vital waves, subtle physical energy waves) or this giving personal-mental form to thought-stuff (or vital movements) from the environing Nature-Force.
The possibilities of the mental being are not limited, it can be the free Witness and Master in its own house. A progressive freedom and mastery over one’s mind is perfectly within the possibilities of anyone who has faith and will to undertake it.