The mind is always in activity, but we do not observe fully what it is doing, but allow ourselves to be carried away in the stream of continual thinking. When we try to concentrate, this stream of self-made mechanical thinking becomes prominent to our observation. It is the first normal obstacle (the other is sleep during meditation) to the effort for yoga.
The best thing to do is to realize that the thought-flow is not yourself; it is not you who are thinking, but thought that is going on in the mind. It is Prakriti with its thought energy that is raising all this whirl of thought in you, imposing it on the Purusha. You as the Purusha must stand back as the witness observing the action, but refusing to identify yourself with it. The next thing is to exercise a control and reject the thoughts – though sometimes by the very act of detachment the thought-habit falls away or diminishes during the meditation and there is a sufficient silence or at any rate a quietude which makes it easy to reject the thoughts that come and fix oneself on the object of meditation. If one becomes aware of the thoughts as coming from outside, from the universal Nature, then one can throw them out before they reach the mind; in that way the mind finally falls silent. If neither of these things happens, a persistent practice of rejection becomes necessary – there should be no struggle or wrestling with the thought, but only a quiet self-separation and refusal. Success does not come at first, but if consent is constantly withheld, the mechanical whirl eventually ceases and begins to die away and one can then have at will an inner quietude or silence.
It should be noted that the result of the yogic processes is not, except in rare cases, immediate and one must apply the will-patience till they give a result which is sometimes long in coming if there is much resistance in the outer nature.
Growing within – The psychology of Inner Development
(Compilation of the works of Sri Aurobindo)