This passage is easier to understand if one reads first this passage:
We see then what we mean precisely by the understanding and by that higher faculty which we may call for the sake of convenience the ideal faculty and which stands to the developed intellect much in the same relation as that intellect stands to the half-animal reason of the undeveloped man. It becomes evident also what is the nature of the purification which is necessary before the understanding can fulfil rightly its part in the attainment of right knowledge. All impurity is a confusion of working, a departure from the dharma, the just and inherently right action of things which in that right action are pure and helpful to our perfection and this departure is usually the result of an ignorant confusion of dharmas in which the function lends itself to the demand of other tendencies than those which are properly its own.
The first cause of impurity in the understanding is the intermiscence of desire in the thinking functions, and desire itself is an impurity of the Will involved in the vital and emotional parts of our being. When the vital and emotional desires interfere with the pure will-to-know, the thought-function becomes subservient to them, pursues ends other than those proper to itself and its perceptions are clogged and deranged. The understanding must lift itself beyond the siege of desire and emotion and, in order that it may have perfect immunity, it must get the vital parts and the emotions themselves purified. The will to enjoy is proper to the vital being but not the choice or the reaching after the enjoyment which must be determined and acquired by higher functions; therefore the vital being must be trained to accept whatever gain or enjoyment comes to it in the right functioning of the life in obedience to the working of the divine Will and to rid itself of craving and attachment. Similarly the heart must be freed from subjection to the cravings of the lifeprinciple and the senses and thus rid itself of the false emotions of fear, wrath, hatred, lust, etc. which constitute the chief impurity of the heart. The will to love is proper to the heart, but here also the choice and reaching after love have to be foregone or tranquillised and the heart taught to love with depth and intensity indeed, but with a calm depth and a settled and equal, not a troubled and disordered intensity. The tranquillisation and mastery of these members is a first condition for the immunity of the understanding from error, ignorance and perversion. This purification spells an entire equality of the nervous being and the heart; equality, therefore, even as it was the first word of the path of works, so also is the first word of the path of knowledge.
The second cause of impurity in the understanding is the illusion of the senses and the intermiscence of the sense-mind in the thinking functions. No knowledge can be true knowledge which subjects itself to the senses or uses them otherwise than as first indices whose data have constantly to be corrected and overpassed. The beginning of Science is the examination of the truths of the world-force that underlie its apparent workings such as our senses represent them to be; the beginning of philosophy is the examination of the principles of things which the senses mistranslate to us; the beginning of spiritual knowledge is the refusal to accept the limitations of the sense-life or to take the visible and sensible as anything more than phenomenon of the Reality.
Equally must the sense-mind be stilled and taught to leave the function of thought to the mind that judges and understands. When the understanding in us stands back from the action of the sense-mind and repels its intermiscence, the latter detaches itself from the understanding and can be watched in its separate action. It then reveals itself as a constantly swirling and eddying undercurrent of habitual concepts, associations, perceptions, desires without any real sequence, order or principle of light. It is a constant repetition in a circle unintelligent and unfruitful. Ordinarily the human understanding accepts this undercurrent and tries to reduce it to a partial order and sequence; but by so doing it becomes itself subject to it and partakes of that disorder, restlessness, unintelligent subjection to habit and blind purposeless repetition which makes the ordinary human reason a misleading, limited and even frivolous and futile instrument. There is nothing to be done with this fickle, restless, violent and disturbing factor but to get rid of it whether by detaching it and then reducing it to stillness or by giving a concentration and singleness to the thought by which it will of itself reject this alien and confusing element.
A third cause of impurity has its source in the understanding itself and consists in an improper action of the will to know. That will is proper to the understanding, but here again choice and unequal reaching after knowledge clog and distort. They lead to a partiality and attachment which makes the intellect cling to certain ideas and opinions with a more or less obstinate will to ignore the truth in other ideas and opinions, cling to certain fragments of a truth and shy against the admission of other parts which are yet necessary to its fullness, cling to certain predilections of knowledge and repel all knowledge that does not agree with the personal temperament of thought which has been acquired by the past of the thinker. The remedy lies in a perfect equality of the mind, in the cultivation of an entire intellectual rectitude and in the perfection of mental disinterestedness. The purified understanding as it will not lend itself to any desire or craving, so will not lend itself either to any predilection or distaste for any particular idea or truth, and will refuse to be attached even to those ideas of which it is the most certain or to lay on them such an undue stress as is likely to disturb the balance of truth and depreciate the values of other elements of a complete and perfect knowledge.
An understanding thus purified would be a perfectly flexible, entire and faultless instrument of intellectual thought and being free from the inferior sources of obstruction and distortion would be capable of as true and complete a perception of the truths of the Self and the universe as the intellect can attain. But for real knowledge something more is necessary, since real knowledge is by our very definition of it supra-intellectual. In order that the understanding may not interfere with our attainment to real knowledge, we have to reach to that something more and cultivate a power exceedingly difficult for the active intellectual thinker and distasteful to his proclivities, the power of intellectual passivity. The object served is double and therefore two different kinds of passivity have to be acquired.
In the first place we have seen that intellectual thought is in itself inadequate and is not the highest thinking; the highest is that which comes through the intuitive mind and from the supramental faculty. So long as we are dominated by the intellectual habit and by the lower workings, the intuitive mind can only send its messages to us subconsciously and subject to a distortion more or less entire before it reaches the conscious mind; or if it works consciously, then only with an inadequate rarity and a great imperfection in its functioning. In order to strengthen the higher knowledge-faculty in us we have to effect the same separation between the intuitive and intellectual elements of our thought as we have already effected between the understanding and the sense-mind; and this is no easy task, for not only do our intuitions come to us incrusted in the intellectual action, but there are a great number of mental workings which masquerade and ape the appearances of the higher faculty. The remedy is to train first the intellect to recognise the true intuition, to distinguish it from the false and then to accustom it, when it arrives at an intellectual perception or conclusion, to attach no final value to it, but rather look upward, refer all to the divine principle and wait in as complete a silence as it can command for the light from above. In this way it is possible to transmute a great part of our intellectual thinking into the luminous truth-conscious vision, — the ideal would be a complete transition, — or at least to increase greatly the frequency, purity and conscious force of the ideal knowledge working behind the intellect. The latter must learn to be subject and passive to the ideal faculty.
But for the knowledge of the Self it is necessary to have the power of a complete intellectual passivity, the power of dismissing all thought, the power of the mind to think not at all which the Gita in one passage enjoins. This is a hard saying for the occidental mind to which thought is the highest thing and which will be apt to mistake the power of the mind not to think, its complete silence for the incapacity of thought. But this power of silence is a capacity and not an incapacity, a power and not a weakness. It is a profound and pregnant stillness. Only when the mind is thus entirely still, like clear, motionless and level water, in a perfect purity and peace of the whole being and the soul transcends thought, can the Self which exceeds and originates all activities and becomings, the Silence from which all words are born, the Absolute of which all relativities are partial reflections manifest itself in the pure essence of our being. In a complete silence only is the Silence heard; in a pure peace only is its Being revealed. Therefore to us the name of That is the Silence and the Peace.
— Sri Aurobindo, The Synthesis of Yoga, pp. 312-16