On the one hand, Aristotle is clear that his account of definition is only appropriate for expressing the essence of physical beings (and mathematical things). On the other hand, the only natural thing that we can define perfectly is the human animal, who, oddly enough, is formally definable only through a power that Aristotle repeatedly proves is non-physical. Reason, the one positive, formal specific difference in a physical substance is not physical. What to make of this?
1.) We could see the genus/ species structure not as biological but as anthropological. Man alone is defined because definition as such is only appropriate to the ontological structure of human persons. We have only found one definition because the criteria of termination for the thing can be satisfied only at the limit of nature and not at the subordinate grades.
2.) We could see definition as requiring introspection. Only what is present to itself can define itself. Perhaps each choir of angels can define itself, the lower choirs, and human persons. By this account, persons can define only themselves and what they themselves give formal being to: artifacts and (perhaps) mathematicals.
2b.) We could see all causes as known only by introspection, and that definition as a sort of cause. This is my idiosyncratic reading of Hume, who denies the possibility of knowing causes in the first book of the Treatise (which seems to focus on our knowledge of nature) and then speaks about causes ad nauseam in the second book (which deals with the sort of moral facts one can know by introspection.) I’ll continue to believe this crazy idea until Brandon refutes it.
3.) We could see man as a paradigm for what is definable. Banez seems to understand man in this way when, in his commentary on the Fourth Way, he objects that the premises of the argument would mean that man is the cause of all animals, but then proceeds to simply say that man is such a thing.
4.) We could just deny that we lack definitions of other natural things. True, we have no good definition of any animal or plant, but we have a pretty good definition of malaria, diabetes, acceleration, etc. But this seems just wrong. We don’t have a good definition of a malaria parasite, only an account of how it affects us and some general essential facts that fall short of a species making difference. For acceleration, see #2.