There is a story told in Arabia that the angels descended from Heaven to earth and cut open the breast of the Prophet; they took away something that was to be removed from there, and then the breast was made as before. It is a symbolical expression, which gives to a Sufi a key to the secret of human Life.

What closes the doors of the heart is fear, confusion, depression, spite, discouragement, disappointment, and a troubled conscience; and when that is cleared away, the doors of the heart open.
The opening of the breast, really speaking, is the opening of the heart.
The sensation of joy is felt in the center of the breast, also the heaviness caused by depression.
Therefore as long as the breast remains choked with anything, the heart remains closed.
When the breast is cleared from it, the heart is open.
It is the open heart which takes the reflection of all impressions coming from outside.
It is the open heart which can receive reflections from the Divine Spirit within.
It is the openness of heart, again, which gives power and beauty to express oneself; and
if it is closed, a man, however learned, cannot express his learning to others.

This symbolical legend explains also what is necessary in the life of man to allow the plant of divine love to grow in his heart. It is to remove that element which gives the bitter feeling. Just as there is a poison in the sting of the scorpion, and as there is a poison in the teeth of the snake, so there is poison in the heart of man, which is made to be the shrine of God. But God cannot arise in the shrine which is as dead by its own poison; it must be purified first, and made real, for God to arise. The heart [of the Prophet] who had to sympathize with the whole world was thus prepared, that the drop of that poison which always produces contempt, resentment, and ill feeling against another, was destroyed first.

So many talk about the purification of heart, and so few really know what it is. Some say to be pure means to be free from all evil thought of bitterness against another. No one with sense and understanding would like to keep a drop of poison in his body, and how ignorant it is on the part of man when he keeps and cherishes a bitter thought against another in his heart. If a drop of poison can cause the death of the body, it is equal to a thousand deaths when the heart retains the smallest thought of bitterness.

In this legend the cutting open of the breast is the cutting open of the ego, which is as a shell over the heart. And the taking away of that element is that every kind of thought or feeling against anyone in the world has been taken away, and the breast, which means the heart, is filled with love alone, which is the real life of God.