 One example of this subordination is found in the relation between the science of geometry and its subordinate sciences. Geometry is the part of speculative philosophy dedicated to the study of limited magnitude (finite limited quantity abstracted from natural matter), its parts, and their properties (DOS 64–65). Astronomy and optics are two such subordinated sciences. Astronomy is a speculative science that has as its object the magnitude of celestial bodies and the distance between them (DOS 72), whereas optics is defined as the speculative science that has as its object the diverse modes of vision, focusing on the nature of visual rays. It belongs to the genus that is the subject matter of geometry because the visual rays propagate rectilinearly (DOS 77).