IV.

Nature

Aristotle’s philosophical writings bear witness to his interest in the natural world. Perhaps the most fundamental part of his philosophy is his analysis of natural or living things, that is, plants and animals, including human beings. In Aristotle’s view, the natural world is made up of individual plants and animals occurring in fixed natural kinds or species. He denied that the natural world is the product of a historical creation or the result of some evolutionary process. The defining characteristic of natural or living things is that they are subject to change: the regular processes of birth, growth, development, and decay that can be observed in the natural world.

Aristotle argued that what explains these natural recurring processes of change is the nature of a living thing such as a plant or animal. Natural things, which exist in nature, have a nature—an internal dynamic principle responsible for the specific pattern of development that each member of a natural species undergoes—consisting of two aspects: a material aspect and a formal aspect. The material aspect is the material of which the plant or animal is composed, and which enables the relevant processes of development to occur, that is, the “matter” of the plant or animal. The formal aspect of the nature of a living thing is its identity as the specific kind of thing it is, which, in combination with the requisite matter, governs the development of the individual as a member of the natural species. This formal aspect is the “form” of the plant or animal. So, if one seeks to understand the development of an acorn into an oak tree, one must understand not only the material processes involved, but also that these material processes occur precisely because they are the material processes necessary for the development of an acorn into an oak tree, and constitute its development as such. For Aristotle, the developmental processes of an acorn could only be understood in relation to the fact that the acorn is developing into an oak tree, that is, into a mature member of the species oak. His analysis of natural processes was thus teleological, since natural processes can only be explained in terms of the end or goal to which they are directed.

From this analysis of the nature of a living thing came Aristotle’s account of the explanatory factors involved in natural change. He identified four such factors, each of which involved either the material or formal aspect of the nature of a living thing. They are (1) the “material cause”, the matter of a living thing; (2) the “formal cause”, its form; (3) the “efficient cause” or the “origin of the change”, the male parent of the living thing in question, which transmits the form in reproduction and thereby initiates the developmental process; (4) the “final cause”, the fully developed living thing, which, as an adult member of its species, has achieved the end to which its earlier processes of development were directed. This end is the realization of form—the identity of the living thing as a member of its species—in the appropriate matter. Of these four explanatory factors, the third—the “efficient cause”—is most readily confused with more modern notions of causation. The requirement that each natural process of development be initiated by an adult member of the species in question is better understood as evidence of Aristotle’s commitment to supposing that the chicken must have come before the egg. His reasons for supposing this were complex, and are set out in Book 8 of the Physics.