Most of Aristotle's work in biology was devoted to zoology. In Aristotle's study of biology the doctrine of teleology is particularly prominent. This doctrine, that the form of natural objects is determined by their final ends or purposes, has frequently been misunderstood as an assertion that there is a universal design in nature. Aristotle simply insists that the structure and the behavior of things also has to be understood as contributing to their individual being and function. Whereas sciences deal with particular kinds of beings, metaphysics is concerned with beings as such.
Substances are prior to nonsubstances because qualities or quantities are determined by substances.
Such substances as God may, however, lack quantities and qualities. Hence an account of beings is, in the first place, an account of substances. Initially, it appears that substances are objects, like the everyday objects, but closer consideration shows that the primary individuals are the forms or essences of particular objects. To understand substance, therefore, it is necessary to consider immaterial substantial forms, ultimately God. Only then can humans understand what it is to be a substance, and what it is to exist.