Some of you may have been drawn into an argument Kleiner and I have been having about the role of teleological explanation in biology. Often, teleological explanation gets described in such a way as to imply that current states of a system should be explained by later states of a system: e.g., that an acorn grows into an oak because that later stage, being an oak, somehow pulls the little acorn in that direction. But that is evidently a mistaken view of how teleological explanation is supposed to work.
Well, how then is it supposed to work? In doing a little research, I came across an older essay (1970) by biologist Franciso Ayala (who lectured on our campus some few years ago) entitled, appropriately enough, “Teleological explanations in evolutionary biology,” and published in the journal Philosophy of Science. I’m going to offer an extended excerpt that I find very clear and helpful:
[…] In this generic sense, teleological explanations are those explanations where the presence of an object or a process in a system is explained by exhibiting its connection with a specific state or property of the system to whose existence or maintenance the object or process contributes. Teleological explanations require that the object or process contribute to the existence of a certain state or property of the system. Moreover, they imply that such contribution is the explanatory reason for the presence of the process or object in the system. It is appropriate to give a teleological explanation of the operation of the kidney in regulating the concentration of salt in the blood, or of the structure of the hand obviously adapted for grasping. But it makes no sense to explain teleologically the falling of a stone, or a chemical reaction.
There are at least three categories of biological phenomena where teleological explanations are appropriate, although the distinction between the categories need not always be clearly defined. These three classes of teleological phenomena are established according to the mode of relationship between the object or process and the end-state or property that accounts for its presence.
(1) When the end-state or goal is consciously anticipated by the agent. This is purposeful activity and it occurs in man and probably in other animals. I am acting teleologically when I pick up a pencil and paper in order to express in writing my ideas about teleology. A deer running away from a mountain lion, or a bird building its nest, has at least the appearance of purposeful behavior.
(2) In connection with self-regulating or teleonomic systems, when there exists a mechanism that enables the system to reach or to maintain a specific property in spite of environmental fluctuations. The regulation of body temperature in mammals is of this kind. In general the homeostatic reactions of organisms belong to this category of teleological phenomena. Two types of homeostasis are usually distinguished by biologists-physiological and developmental homeostasis, although intermediate situations may exist. Physiological homeostatic reactions enable the organism to maintain certain physiological steady states in spite of environmental shocks. The regulation of the composition of the blood by the kidneys, or the hypertrophy of a structure like muscle due to strenuous use, are examples of this type of homeostasis. Developmental homeostasis refers to the regulation of the different paths that an organism may follow in its progression from zygote to adult. Self-regulating systems or servo-mechanisms built by man are teleological in this second sense. The simplest example of such servo-mechanisms is a thermostat unit that maintains a specified room temperature by turning on and off the source of heat. Self-regulating mechanisms of this kind, living or man-made, are controlled by a feed-back system of information.
(3) In reference to structures anatomically and physiologically designed to perform a certain function. The hand of man is made for grasping, and his eye for vision. Tools and certain types of machines made by man are teleological in this sense. A watch for instance, is made to tell time, and a faucet to draw water. The distinction between this and the previous category of teleological systems is some-times blurred. Thus the human eye is able to regulate itself within a certain range to the conditions of brightness and distance so as to perform its function more effectively.
Teleological mechanisms in living organisms are biological adaptations. They have arisen as a result of the process of natural selection. The adaptations of organisms-whether organs, homeostatic mechanisms, or patterns of behavior-are explained teleologically in that their existence is accounted for in terms of their contribution to the reproductive fitness of the population. As explained above, a feature of an organism that increases its reproductive fitness will be selectively favored. Given enough time it will extend to all the members of the population.