Abstract: The relationship between ontology and theology in Aristotle’s Metaphysics has always been a controversial issue among scholars. Since the 20th century, the focal meaning interpretation represented by Patzig-Frede has provided a classical scheme to tackle this problem, and the causal relationship between the special substance and the general substance in this scheme needs to be further explained. Book B, as the planned book of Metaphysics, provides a key idea for this: as a science of inquiry into arche and cause, metaphysics deals with how arche is generally the cause of being, and the necessary condition of arche is to be the substance. Metaphysics could thus simultaneously deal with the discussion of being and substance, and how substance, as arche, is the cause of other beings, thus unifying Aristotle’s inquiry in a single science.