412. What has been said may be seen in a kind of image and thus corroborated by the correspondence of the heart with love and of the lungs with the understanding (of which above). For if the heart corresponds to love, its determinations, which are arteries and veins, correspond to affections, and in the lungs to affections for truth; and as there are also other vessels in the lungs called air vessels, whereby respiration is carried on, these vessels correspond to perceptions. It must be distinctly understood that the arteries and veins in the lungs are not affections, and that respirations are not perceptions and thoughts, but that they are correspondences, that is, they act correspondently or synchronously; likewise that the heart and the lungs are not the love and understanding, but correspondences: and inasmuch as they are correspondences the one can be seen in the other. Whoever from anatomy has come to understand the whole structure of the lungs can see clearly, when he compares it with the understanding, that the understanding does not act at all by itself, does not perceive nor think by itself, but acts wholly by affections which are of love. These, in the understanding, are called affection for knowing, for understanding, and for seeing truth (which have been treated of above). For all states of the lungs depend on the blood from the heart and from the vena cava and aorta; and respirations, which take place in the bronchial branches, proceed in accordance with the state of those vessels; for when the flow of the blood stops, respiration stops. Much more may be disclosed by comparing the structure of the lungs with the understanding, to which the lungs correspond; but as few are familiar with anatomical science, and to try to demonstrate or prove anything by what is unknown renders it obscure, it is not well to say more on this subject. By what I know of the structure of the lungs I am fully convinced that love through its affections conjoins itself to the understanding, and that the understanding does not conjoin itself to any affection of love, but that it is reciprocally conjoined by love, to the end that love may have sensitive life and active life. But it must not be forgotten that man has a twofold respiration, one of the spirit and another of the body; and that the respiration of the spirit depends on the fibers from the brains, and the respiration of the body on the blood-vessels from the heart, and from the vena cava and aorta. It is evident, moreover, that thought produces respiration; it is evident, also, that affection, which is of love, produces thought, for thought without affection is precisely like respiration without a heart, a thing impossible. From this it is clear that affection, which is of love, conjoins itself to thought, which is of the understanding (as was said above), in like manner as the heart does in the lungs.