Change, motion, and explanation › What Aristotle means by nature

Book II begins by distinguishing things that exist by nature from products of craft. Animals, plants, and the elementary bodies have an internal source of change and rest. A bed, considered specifically as a bed, does not. Its wood can nevertheless have natural capacities, so the distinction depends on the respect in which we describe the thing (Physics II.1).

A wooden chair may decay. That does not show that its furniture design possesses its own natural life cycle in Aristotle's sense. The material changes because of the capacities of the wood. Conversely, a plant's growth belongs to the kind of living thing it is. This is an illustration of Aristotle's distinction, not a claim that he analysed chairs using our manufacturing methods.

An internal principle is not the same as complete independence from external conditions. A plant can require water and suitable surroundings. Nor does the definition say that every natural body consciously directs itself. The wording includes being moved and being at rest. Reading "internal" as "needs nothing else" would make the distinction less informative.

Aristotle considers both matter and form as candidates for nature, and gives form priority in an important respect: what has reached its form is more fully the thing in question than what has it only potentially. His example contrasts the material of a bed with the organized living thing that generates another living thing of its kind (Physics II.1). Nature therefore concerns organization as well as material composition.

The student of nature must investigate both. A definition of an organism cannot ignore its material realization, while a list of ingredients alone may fail to explain what makes it the organized thing it is. Aristotle compares the required knowledge with a physician's knowledge of health and the bodily materials in which health is realized (Physics II.2).