مَا بِهِ الشَّيْءُ هُوَ هُوَ
That by which a thing is what it is.
This expression seeks a defining nature rather than a mere label. The point is to identify what is essential to the kind under discussion.
A human can stand, sit, or walk at different moments without ceasing to be human. These changing conditions should not be mistaken for the constitutive nature of the kind.