Species are the best candidates for substancehood because
they have no nonessential properties but they also have thisness.
While Joe and Adam have particular quirks that are true of only
them, the species of human in general has no particular quirks outside
the definition that holds of all humans. The essential properties
of being human, like rationality, do not presuppose the existence
of anything else because rationality is a part of what it is to
be human. These essential properties also make humans distinguishable
from ducks or rocks or trees. Hence, Aristotle reaches the remarkable
conclusion that humans and ducks and rocks and trees are the most
fundamental building blocks of reality. We can imagine particular
humans or ducks or rocks or trees only because these species exist,
and we can only infer general categories such as animal or mineral
or plant from the existence of species that fit within these categories.
