As the soul advances towards the formless, unable to grasp what is without contour or to receive the imprint of reality so diffuse, it fears it will encounter nothingness, and it slips away.
– Plotinus: ‘The Good or The One’ VI, 9 
The divine being is equal to nothing, and in it there is neither image nor form…[Therefore] When the soul…contemplates what consists of images, whether that be an angel’s image or its own, there is for the soul some thing lacking. Even if the soul contemplates God…the soul lacks something. But if all images are detached from the soul, and it contemplates only the Simple One, then the soul’s naked being finds the naked, formless being of the divine unity.
– Meister Eckhart: ‘Die Deutsche Werke’
The Sufis sought to lose what they currently perceived as labels, knowledge, concepts and to become empty (nothing) and attain the state of “void”; to attain a zero point so that they could become related to any state of being and achieve “everythingness”. Just as the discovery of zero in mathematics made the system possible, so too in the art of rebirth, the discovery of a state of “nothingness” (the void or emptiness) makes final integration a possibility.
– A. Reza Arasteh: ‘Final Integration in the Adult Personality’, 1965.