Its subject matter is the dualistic world and dualistic experiences. Generally it is so with all jIvas.
It thickens by the dirt of experience and stays like the dirty and greasy stain attaching itself to cooking vessels. This is a matter of the inner organ.
The heart that I speak of, on the other hand, is again not the physical organ on the left side of the chest of the human body.
Nor is it the anAhata-chakra, located in the dead centre of the chest, in the suShumnA nAdi that is within the spine.