 Beyond its role in explaining motivation, xing helps explain the organization of the world into individuals and kinds. In a series of passages arguing with a rival named Gaozi, Mengzi shows that whatever would explain the natures of things must have its own dynamism and directionality, and it must explain the specific differences between kinds of things (6A1–3). On the one side, xing differentiates things in terms of coherent patterns of force, providing a contextual and provisional basis for individuation. One can refer to the xing of a human being but also the xing of the human mouth (6A7), to the xing of a mountain ecosystem (6A8) or the xing of a single tree (6A1). On the other side, xing was generally used as a species concept—things of the same kind have the same xing. Mengzi’s ethical philosophy is based on his belief that human beings share certain ways of responding to the world, all having the same xing. The status of species, though, was a point of controversy, linked to the question of whether or not all human beings could be held to the same standards. A more radical line of thought took each individual as having its own unique xing, a view rooted in parts of the Zhuangzi and developed later by Guo Xiang (?–312 CE).