On Breath

​2 On the other hand, that which is derived from respiration, as Aristogenes thinks—for he thinks that the spirit also is nourished by the air being digested in the lung, and that this is distributed into the vessels, while the residue is sent out again—presents more difficulties. For, in the first place, by what is the digestion performed? For it is reasonable that it is by the spirit itself, just as with other things. But this very thing is absurd, if it does not differ from the outside air; for in that case would the heat digest it? And indeed, it is reasonable that it is thicker, being accompanied by moisture from the vessels and the whole masses, so that the digestion would be into the bodily state. And the residue, if indeed it becomes thinner, is not plausible. And the speed of the digestion is also unreasonable. For exhalation follows immediately after inhalation. What is it, then, that changes and alters it so quickly? For one might especially suppose it to be the heat, and sensation bears witness to this; for the exhaled air is warm. Furthermore, if that which is digested is in the lung and the artery, the power of the heat must be in these; which is not what they say, but rather that the nourishment is heated in the movement of the spirit. But if it is as it were drawn from something else, or receives it from something that moves it, this is still more wonderful; and at the same time, it would not be the first mover itself. Furthermore, respiration is only as far as the lung, as they themselves say, but the connate spirit is throughout the whole. But if it is distributed from this both to the lower parts and to the others, how is the digestion so rapid? For this is more wonderful and a greater difficulty; for it does not immediately send this digested air to the lower parts. And yet this would seem to be necessary, if digestion takes place in the lung and the lower parts also share in respiration; but even so, what happens is still more difficult and paradoxical; for digestion would occur as it were by mere passage and contact. And this too is unreasonable and more theoretical, if the passage for the nourishment and the residue is the same. But if it is through some other of the internal passages, the same arguments apply as before, unless one were to say this: that residue does not come from all nourishment nor in all things, just as it does not in plants, since indeed it is not possible to find residue for each of the parts of the body, except perhaps for the whole. But is the growth of the vessels the same as that of the other parts, and as these dilate and separate, the air that flows in and out becomes more? But if some necessary thing is present within, this is the very thing being sought: what is the natural one? And how this becomes greater in a healthy way would be clear from this. And for those creatures that do not respire, what is the nourishment of the connate spirit and what is its growth? For it can no longer be from the outside for them. But if it is from the internal parts and the common nourishment, it is reasonable for them too; for similar things come from the same things and in the same way, unless indeed for them too it is from the outside, in the same way as they perceive odors. But surely no such thing as respiration occurs in them. Concerning which one might be puzzled whether it is true, bringing forward this very point and the attraction of nourishment (for it is at the same time an attraction of spirit), and furthermore arguing on behalf of cooling, on the ground that they too require it. But if it occurs to them through the diaphragm, in this way it is clear that the entrance of air also takes place; so that it is something similar to respiration. Except that it is not defined what the attraction is and by what it is caused, or if it is not an attraction, how the entrance occurs, unless indeed it is spontaneous. This subject, then, also requires consideration by itself. And for aquatic creatures, what is the nourishment and growth of the connate spirit? For apart from the fact that they do not respire, we say that there is no air at all in the liquid. It remains, then, that it is through nourishment, though not in the same way for all, or else those other non-aquatic creatures also are nourished through nourishment; for one of these three must be necessary. And these are the matters concerning the growth and nourishment of the spirit.