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.