We must withdraw every thing and thought from the mind except this single thought of trying to achieve the absence of what is not the Absolute. This is called Gnana Yoga: "Neti, Neti" (It is not this), as Shankara called it. And he must go on with this negative elimination until he reaches the stage where a great Void envelops him. If he can succeed in holding resolutely to this Void in sustained concentration--and he will discover it is one of the hardest things in the world to do so--he will abruptly find that it is not a mere mental abstraction but something real, not a dream but the most concrete thing in his experience. Then and then only can he declare positively, "It is This." For he has found the Overself.
-- Notebooks Category 23: Advanced Contemplation > Chapter 8: The Void As Contemplative Experience > # 118
-- Perspectives > Chapter 23: Advanced Contemplation > # 76