Tertullian

“ Thus, if a man likewise be designated a wild beast or a harmless one, there is not for all that an identity of soul. Now the similarity of nature is even then observed, when dissimilarity of substance is most conspicuous: for, by the very fact of your judging that a man resembles a beast, you confess that their soul is not identical; for you say that they resemble each other, not that they are the same. This is also the meaning of the word of God (which we have just quoted) : it likens man to the beasts in nature, but not in substance. ”

“ A stone or a piece of iron is the substance: the hardness of the stone and the iron is the nature of the substance. Their hardness combines objects by a common quality; their substances keep them separate. ”

“ It must needs be, that every individual body of whatever size is filled up by the soul, and that the soul is entirely covered by the body. How, therefore, shall a man’s soul fill an elephant? How, likewise, shall it be contracted within a gnat? ”

“ For although some men are compared to the beasts because of their character, disposition, and pursuits (since even God says, “Man is like the beasts that perish” [4] ) , it does not on this account follow that rapacious persons become kites, lewd persons dogs, ill-tempered ones panthers, good men sheep, talkative ones swallows, and chaste men doves, as if the selfsame substance of the soul everywhere repeated its own nature in the properties of the animals (into which it passed) . Besides, a substance is one thing, and the nature of that substance is another thing ”

“ Then opposed to spirit are those creatures which seem to have no respiration, being unfurnished with lungs and windpipes, such as gnats, ants, moths, and minute things of this sort. Opposed, moreover, to air are those creatures which always live under ground and under water, and never imbibe air—things of which you are more acquainted with the existence than with the names. ”