"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: "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 consist of its "quiddity"/essence.