Now the condition of the heart is this, that it carry nothing. It cannot contain two things, but the unity which it holds in love may be simple or complex, very large or greater than all the universe. This is the explanation of the teaching of Upanishad. In the Upanishads, Atman often means the same as heart-essence, and this heart-essence grasps both great and small, but whatever it holds, it holds nothing else.
For that reason Sufis practice heart-concentration, first to restore to the heart its faculty of grasping and then to prepare the heart for grasping that which alone does it good; that is to say, to grasp God, to hold God. All Sufi practices have this object in view.