Now, souls in general are handled very badly indeed in fantasy fiction,often like a detachable thing with feathers, without even the charm of primitive tales of animals who are really people's "souls." And they have all sorts of furor about people without souls, as if a body without a soul was not a corpse.
But this particular author was laying out things souls can be, and among them were:
- The animating principle of all living things
- The animating principle of things with senses
- The animating principle of rational beings
But the author then went on to discuss them as if the principles were mutually exclusive. That if you believed that souls had faculties, you could not believe that only certain souls had some, and other souls did without, so if you believed plants had some kind of soul, you had to believe that humans had no more of a soul. I really doubt he could have believed that, given that tripartite distinction was indeed believed in medieval times. The vegetative soul, the most basic kind, had only life and growth; the sensitive soul had all a vegetative soul's faculties, and the five senses, and the ability of locomotion; the rational soul, all of a sensitive soul's, and rational judgment.
When doing your metaphysical speculation for, or about, a novel, it really would be wiser to get the actual metaphysics you're dealing with down pat, first.