412. What has been said may be seen in some resemblance and so confirmed by the correspondence of the heart with love and of the lungs with the understanding (of which above). For since the heart corresponds to love, its extensions 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, by which breathing is effected, these vessels correspond to perceptions. It must be correctly 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, for their action is correspondent or synchronous; similarly, that the heart and lungs are not love and the understanding, but correspondences; and since they are correspondences, the one may be seen in the other. Anyone who has acquired a knowledge of the structure of the lungs from a study of anatomy, and makes comparison with the understanding, can see clearly that the understanding does nothing of itself, does not perceive nor think from itself, but it does everything from love's affections, which in the understanding are called affections for knowing, understanding, and seeing truth (and were treated of above). For all the conditions of the lungs depend upon the blood from the heart and from the vena cava and aorta; and breathing, which takes place in the bronchial branches, goes on according to those conditions; for when the inflow of blood stops, breathing stops. Much more can yet be disclosed by comparing the structure of the lungs with the understanding, to which the lungs correspond; but as a knowledge of anatomy is confined to few and to demonstrate or confirm anything by the unknown renders it obscure, it is not advisable to say more on the matter. From my knowledge of the structure of the lungs I am fully convinced that love conjoins itself to the understanding through its affections, and that the understanding does not conjoin itself to any affection of love, but that it is conjoined reciprocally by love because of the end, that love may have sensate and active life. But indeed it ought to be known that man has a twofold respiration, one of the spirit, the other of the body; and that the breathing of the spirit depends on the fibres from the brains, and the breathing of the body on the blood vessels from the heart and from the vena cava and aorta. Moreover, it is evident that thought draws out the respiration, and it is evident also that love's affection draws out thought; for thought apart from affection is precisely like respiration without the heart-an impossibility. Hence it is clear that love's affection conjoins itself to thought which is of the understanding, as said above, in the same way as the heart does in the lungs.