That the Souls of Dumb Animals are not Immortal

No activity of the sentient part can have place without a body. But in the souls of dumb animals we find no activity higher than the activities of the sentient part. That animals neither understand nor reason is apparent from this, that all animals of the same species behave alike, as being moved by nature, and not acting on any principle of art: for every swallow makes its nest alike, and every spider its web alike. Therefore there is no activity in the soul of dumb animals that can possibly go on without a body.*

2. Every form separated from matter is actually understood. Thus the active intellect makes impressions actually understood, inasmuch as it abstracts them. But if the soul of a dumb animal remains after the body is gone, it will be a form separated from matter. Therefore it will be form actually understood. But "in things separated from matter understanding and understood are the same" (De Anima, III, iv, 13). Therefore the soul of a dumb animal will have understanding, which is impossible.*

In everything that is apt to arrive at any perfection, there is found a natural craving after that perfection: for good is what all crave after, everything its own good. But in dumb animals there is no craving after perpetuity of being except in the form of perpetuity of the species, inasmuch as they have an instinct of generation, whereby the species is perpetuated, -- and the same is found in plants.* But they have not that craving consequent upon apprehension: for since the sentient soul apprehends only what is here and now, it cannot possibly apprehend perpetuity of being, and therefore has no physical craving after such perpetuity.* Therefore the soul of a dumb animal is incapable of perpetuity of being.

2.80, 81 : Arguments of those who wish to prove that the Human Soul perishes with the Body, with Replies to the same
2.83, 84 : Apparent Arguments to show that the Human Soul does not begin with the Body, but has been from Eternity, with Replies to the same