Deeply hidden characteristics in other souls can be perceived by this organ, but their truth depends on the attainment of immunity from the above-mentioned illusions. For this purpose it is necessary that the student should control and dominate everything that seeks to influence him from outside. He should reach the point of really receiving no impressions beyond those he wishes to receive. This can only be achieved by the development of a powerful inner life; by an effort of the will he only allows such things to impress him to which his attention is directed, and he actually evades all impressions to which he does not voluntarily respond. If he sees something it is because he wills to see it, and if he does not voluntarily take notice of something it is actually non-existent for him.
Rudolf Steiner, How to Know Higher Worlds [Amazon, Bookshop, Publisher, Local Library]