Chapter IV.
To teach the mutual dependence of water and fish, and what kind of dependence there should be between parents and children. How water provides fish with the ability to breathe.
Consider how good water is as a mother. You, O man, have taught about the abandonment of fathers by their sons, separations, hatred, offenses; learn what the necessity is between parents and children. Fish cannot live without water, nor can they be separated from the company of their parent, nor can they be distinguished from their foster mother's care; and this happens by a certain nature, that when separated they die immediately. Neither, indeed, do they live by the breath of this air, as all things do, because the nature of drawing in breath and breathing out is not sufficient for them; otherwise they could not always live under water, not having the infusion of breath. What the spirit is to us, water is to them. Just as the spirit serves us as the substance of life-giving, so does water serve them. With the supply of spirit cut off, we cannot remain alive for even a short time without the vital spirit, and we are immediately extinguished. Likewise, fish removed from water cannot exist without the substance of their own life.
11. And the reason is clear, because in us the lung receives breath through the more spacious cavities of the chest; and since it is itself permeable to most pores, it cools the internal heat by the infusion of breath. For just as the chest receives nourishment, so it also separates excess food and healthy juices and distinguishes the blood. The lung becomes permeable, so that the breath can more easily reach it by aspiration. However, fish have gills, which they now fold and gather, now spread out and open. Therefore, in this collection and filtering process, while water is being taken in, transmitted, and penetrating, it seems to fulfill the function of respiration. Therefore, it is the nature of fish, not common with others; a special purpose, and a certain separate and secret substance for living. That is why they are not nourished, nor do they delight in the touch and caress of human hands like terrestrial animals, even if they live in their own ponds.