An argument for the three-species view

Some contemporary Thomists have the idea that there are exactly three metaphysical species—three kinds differentiated by qualitatively different natures—of living things: plants (maybe broadly understood as non-sentient living things), mere animals, and rational animals.

Here’s a line of thought that yields two-thirds of the view, starting with a premise that most medieval Aristotelians would have accepted:

1. Our (metaphysical) species is rational animal.

2. Therefore, if there were a rational fish, it would be a member of our species.

3. And, a fortiori any rational ape would be a member of our species.

4. So, all rational fish would be the same species as all rational apes.

5. If all rational fish would be the same species as all rational apes, all non-rational fish are the same species as all non-rational apes.

6. The above generalizes from fish to all other animals.

7. So, all rational animals are the same species and all non-rational animals are the same species.

I don’t have an argument for 5, but it seems pretty plausible.

And the claim that all living non-sentients are the same species doesn’t seem implausible given 7.

I myself reject 1.