The differentiation of heaven into two kingdoms, one called the heavenly kingdom and the other the spiritual kingdom, has been presented in its proper chapter above. The heavenly kingdom in general corresponds to the heart and to all the extensions of the heart throughout the body. The spiritual kingdom corresponds to the lungs and to all their extension throughout the body. Further, the heart and the lungs form two kingdoms in us, the heart governing through the arteries and veins and the lungs through the nerve and motor fibers, each involved in every effort and action.
Within each one of us, in the spiritual world of ours that is called our spiritual person, there are also two kingdoms. One is volitional and the other is cognitive, the volitional governing through affections for what is good and the cognitive through affections for what is true. These kingdoms also correspond to the kingdoms of the heart and lungs in the body. The same holds true in the heavens. The heavenly kingdom is heaven’s volitional side, where the good that flows from love rules. The spiritual kingdom is heaven’s cognitive side, where truth rules. These are what correspond to the functions of the heart and the lungs in us.
It is because of this correspondence that “the heart” in the Word means volition and the good of love, while “the breath of the spirit” means understanding and the truth of faith. This is also why we ascribe feelings to the heart, even though they do not reside or originate there.
from Heaven and Hell, Section 95