  A science consists of a genus (genos), what the science is about, and a collection of attributes, what the science says about the genus. The genus or kind is both defined and hypothesized to exist. From his examples (points and lines for geometry), it would seem that the genus is to be understood loosely as the fundamental entities in the science. The attributes are defined but are not hypothesized as existing. One must prove that the attributes belong to various members of the genus. For example, one must prove that triangles exist, e.g., that some [constructible] figures are triangles.