The stillness of the mind is prepared by the process of concentration. In the science of Rajayoga after the heart has been stilled and the mind prepared, the next step is to subjugate the body by means of asan or the fixed and motionless seat. The aim of this fixity is twofold, first the stillness of the body and secondly the forgetfulness of the body. When one can sit still and utterly forget the body for a long period of time, then the asan is said to have been mastered. In ordinary concentration when the body is only comparatively still it is not noticed, but there is an undercurrent of physical consciousness which may surge up at any moment into the upper current of thought and disturb it. The Yogin seeks to make the forgetfulness perfect. In the higher processes of concentration this forgetfulness reaches such a point that the bodily consciousness is annulled and in the acme of the samadhi a man can be cut or burned without being aware of the physical suffering. Even before the concentration is begun the forgetfulness acquired is sufficient to prevent any intrusion upon the mind except under a more than ordinarily powerful physical stimulus. After this point has been reached the Yoga proceeds to the processes of pranayam by which the whole system is cleared of impurities and the pranasakti, the great cosmic energy which lies behind all processes of Nature, fills the body and the brain and becomes sufficient for any work of which man is actually or potentially capable. This is followed by concentration.
The first process is to withdraw the senses into the mind. This is partly done in the ordinary process of absorption of which every thinking man is capable. To concentrate upon the work in hand whether it be a manual process, a train of thought, a scientific experiment or a train of inspiration, is the first condition of complete capacity and it is the process by which mankind has been preparing itself for Yoga. To concentrate means to be absorbed; but absorption may be more or less complete. When it is so complete that for all practical purposes the knowledge of outward things ceases, then the first step has been taken towards Yogic absorption. We need not go into the stages of that absorption rising from pratyahar to samadhi and from the lower samadhi to the higher. The principle is to intensify absorption. It is intensified in quality by the entire cessation of outward knowledge, the senses are withdrawn into the mind, the mind into the buddhi or supermind, the supermind into Knowledge, Vijnana, Mahat, out of which all things proceed and in which all things exist.
It is intensified in quantity or content; instead of absorption in a set of thoughts or a train of intuitions, the Yogin concentrates his absorption on a single thought, a single image, a single piece of knowledge, and it is his experience that whatever he thus concentrates on, he masters, - he becomes its lord and does with it what he wills. By knowledge he attains the mastery of the world. The final goal of Rajayoga is the annulment of seperate consciousness and complete communion with that which alone is whether we call Him Parabrahman or Parameshwara, Existence in the Highest or Will in the Highest, the Ultimate or God.
Excerpt from the book Essays Divine and Human by Sri Aurobindo