“To illustrate the multiple structure of the heart, the protective function of each layer, and the need to keep all of them intact, the author (Hakim at-Tirmidhi) suggests several analogies. Here are a few abridged passages culled from this treatise (The Difference Between the Chest, the Heart, the Inner Heart, and the Kernel of the Heart):
The name ‘heart’ is a general name which refers to its inner stations. There are spaces within and without that [which is commonly known as] heart. [In this respect] the name ‘heart’ resembles the name ‘eye’, since ‘eye’ refers to all that is included within the rim of the eyelids: the white of the eye, the black [of the eye], the pupil, and the light in the pupil. What is external is the container for the inner which lies within it.
The name ‘house’ too is general, since it refers to all that is included within its walls: the rooms, the hall, the courtyard surrounding the rooms, the bedchamber, the store-house. Each of these spaces has a specific function which sets it apart from the rest.
The name ‘almond’ too is general. It includes the outer shell which covers the husk, the kernel, and the oil within the kernel.
The author then comments on the esoteric aspect of this layered physiology:
Know that the higher the knowledge, the more concealed, the more gauarded, the more hidden it is. Laymen, however, use the name ‘heart’ to refer to all its inner spaces.
Then the analogies between the heart, the eye, the house, the holy city of Mecca, the lamp, and the almond are drawn in more detail. Here are some of the statements the author makes:
The chest in relation to the heart is like the white of the eye in relation to the eye, or like the courtyard in relation to the house, or like the container of water in relation to the lamp, or like the outer shell in relation to the almond.
The chest (sadr) is the space into which desires and inclinations enter. This is the domain of the lower self (nafs).
The heart (qalb) lies within the chest, and is like the black of the eye within the white of the eye. This is the abode of the light of faith, humility, piety, love, fear, hope, and content.
The inner heart (fu’ad) is the third station. It is like the pupil of the eye within the black of the eye, or like the kernel within the almond. The fu’ad is the place of Divine knowledge and visions. The inner heart is in the center of the heart, just as the heart is in the center of the chest, like a pearl within a shell.
The kernel of the heart (lubb) is like the light of seeing within the eye, like the light of the lamp within the wick, like the oil concealed within the kernel of the almond.
The external parts protect and cover that which lies within them.
Beyond these there are ever-finer stations, loftier spaces, and more exquisite subtleties. The root of all of this is the light of Unity.
Source: Sara Sviri, The Taste of Hidden Things, Chapter 1 “The Niche of Lights”, pp. 6-7