Sri Aurobindo
Letters of Sri Aurobindo
Volume IV - Part 1
Then for the tumultuous activity of the mind which prevents your concentration. But that or else a more tiresome obstinate grinding mechanical activity is always the difficulty when one tries to concentrate and it takes a long time to get the better of it. That or the habit of sleep which prevents either the waking concentration or the conscious samadhi or the absorbed and all-excluding trance which are the three forms that Yogic concentration takes. But it is surely ignorance of Yoga, its processes and its difficulties that makes you feel desperate and pronounce yourself unfit for ever because of this quite ordinary obstacle. The insistence of the ordinary mind and its wrong reasonings, sentiments and judgments, the random activity of the thinking mind in concentration or its mechanical activity, the slowness of response to the veiled or the initial touch are the ordinary 
obstacles the mind imposes just as pride, ambition, vanity, sex, greed, grasping of things for one’s own ego are the difficulties and obstacles offered by the vital. As the vital difficulties can be fought down and conquered, so can the mental. Only one has to see that these are the inevitable obstacles and neither cling to them nor be terrified or overwhelmed because they are there. One has to persevere till one can stand back from the mind as from the vital and feel the deeper and larger mental and vital Purushas within one which are capable of silence, capable of a straight receptivity of the true Word and Force as of the true silence. If the nature takes the way of fighting down the difficulties first, then the first half of the way is long and tedious and the complaint of the want of the response of the Divine arises. But really the Divine is there all the time, working behind the veil as well as waiting for the recognition of his response and for the response to the response to be possible.