is as it were reported to the eye by this continuous cone of air extended towards it like a staff. In the same way, we hear because the air between the speaker and the hearer is struck in a spherical manner; and is then agitated in waves, resembling the circular eddies which one sees in a cistern when a stone is dropped into it.
Sleep, they say, is produced by a relaxation of the æsthetic energies with reference to the dominant part of the soul. And the causes of the passions they explain to be the motions and conversions which take place in connection with this spirit or soul.
LXXXV. Seed, they define as a thing of a nature capable of producing other things of the same nature as the thing from which it has been separated. And the seed of man, which man emits, is, together with moisture, mixed up with the parts of the soul by that kind of mixture which corresponds to the capacity of the parents. And Chrysippus says, in the second book of his Natural Philosophy, that it is a spirit according to substance; as is manifest from the seeds which are planted in the earth; and which, if they are old, do not germinate, because all their virtue has evaporated. And Sphærus says, that seed proceeds from the entire body, and that that is how it is that it produces all the parts of the body.
They also say that the seed of the female is unproductive; for, as Sphærus says, it is devoid of tone, and small in quantity, and watery.
LXXXVI. They also say that that is the dominant part of the soul which is its most excellent part; in which the imaginations and the desires are formed, and whence reason proceeds. And this place is in the heart.
These then are the doctrines on the subject of natural philosophy entertained by them, which it seems sufficient for us to detail, having regard to