The crescent in the heart suggests the responsiveness of the heart. The crescent represents the responsiveness of the crescent moon to the light of the sun, for naturally it receives the light, which develops it until it becomes the full moon. The principal teaching of Sufism is that of learning to become a pupil, for it is the pupil who has a chance of becoming a teacher, and once a person considers that he is a teacher, his responsiveness is gone. The greatest teachers of the world have been the greatest pupils. It is this principle which is represented by the crescent: the crescent in the heart signifies that the heart which is responsive to the light of God is illuminated.