• The action of a tornado or volcano, however complex, is best explained in mechanistic terms, perhaps including such notions as "equilibrium" and "feedback relationships." On the other hand, the action of a plant as it turns its leaves toward the sun is better understood in terms of goals or functions necessary to the life of the plant, which are accomplished by means of internal programming. Of course, man-made devices such as robots or computers may also exhibit programming, but here the goals originate in human beings, so the action is not self-generated.

In a basic sense, a non-living thing such as a rock, tornado, or volcano need not "do" anything to ensure its own survival. The actions of such entities are fundamentally mechanistic rather than function-oriented. In contrast, living organisms (including humans) must generate certain processes—either internal or external—in order to sustain life. This requirement gives rise to the concept of function, which is applicable not only to living things but also to those inanimate things (such as beaver dams and human tools) that are generated by and support life processes. A function is a process or role that explains an entity's existence and by which it promotes the life of an organism, either directly or indirectly.     

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