Being is divided in two ways: that which is "in itself" (substances), and that which is "in another" (accidents). Substances are things which exist "per se" or in their own right. Accidents are qualities that apply to other things, such as shape or color: "ccidents must include in their definition a subject which is outside their genus." Because they only exist in other things, Aquinas holds that metaphysics is primarily the study of substances, as they are the primary mode of being.