18. But since our nature is poor and in need of supplies for its own maintenance from all quarters, it not only lacks air of its own, and the breath which excites heat, which it imports from without for the preservation of the living being, but the nourishment it finds to fill out the proportions of the body is an importation. Accordingly, it supplies the deficiency by food and drink, implanting in the body a certain faculty for appropriating that which it requires, and rejecting that which is superfluous, and for this purpose too the fire of the heart gives nature no small assistance.