banks. Modern philosophers call it ambiguity. Further, the usage of "definition" that Aquinas gives is the genus of the being, plus a difference that sets it apart from the genus itself. For instance, the Aristotelian definition of "man" is "rational animal"; its genus being animal, and what sets apart man from other animals is his rationality. Being In Thomist philosophy, the definition of a being is "that which is," a principle with two parts: "that which" refers to its quiddity (literally "whatness"), and "is" refers to its esse (Latin "to be"). Quiddity means an essence, form, or nature which may or may not exist; whereas esse refers to existence or reality. That is, a being is "an essence that exists." 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: "[A]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. The Catholic Encyclopedia pinpoints Aquinas' definition of quiddity as "that which is expressed by its definition." The quiddity or form of a thing is what makes the object what it is: "[T]hrough the form, which is the actuality of matter, matter becomes something actual and something individual," and also, "the form causes matter to be." Thus, it consists of two parts: "prime matter" (matter without form), and substantial form, which is what causes a substance to have its characteristics. For instance, an animal can be said to be a being whose matter is its body, and whose soul is its substantial form. Together, these constitute its quiddity/essence. All real things have the transcendental properties of being: oneness, truth, goodness (that is, all things have a final cause and therefore a purpose), etc. Causality Aristotle categorized causality into four subsets in the Metaphysics, which is an integral part of Thomism: (a) refers to the material cause, what a being's matter consists of (if applicable). (b) refers to the formal cause, what a being's essence is. (c) refers to the efficient cause, what brings about the beginning of, or change to, a being. (d) refers to the final cause, what