The word "heart" in metaphysics denotes the main center of the mental plane. The piece of flesh which we term heart is the sensitive part in us which feels the effect of all joy and pain before any other organ; and from this center the breath carries on the work of spreading all energy throughout the physical body. That is why the Sufi works through this center in the physical body when he wishes to impress his absolute self with a certain thought; but high development lies in purifying the five faculties before mentioned by the mystical process and in mastering them.