Posted on behalf of Steve Hays...
Human curiosity leads people to ask what things are for. Teleological explanations answer “What for?” questions by appealing to forward-looking reasons.
Children learn that certain things—a snapped twig, a pattern of pebbles washed up by the tide—are not thought to be explainable in this way. But there is a vast range of phenomena which adults do try to explain teleologically. Children accept these explanations and learn rules for constructing them. Folk acceptability is no guarantee of scientific acceptability, however. Many thinkers have held that the study of purposes is no business of science.
Goal explanations are widely employed in psychology, ethology, and Artificial Intelligence; natural function explanations figure in biology; social function explanations occur in anthropology, sociology, and sociobiology.
Yet there are some whose idealized conception of good science challenges the legitimacy of goal talk, and others who believe that natural function attributions are not wholly objective.
Typically, a functional explanation in biology says that an organ x is present in an animal because x has a function F. What does this mean?
Some philosophers maintain that an activity of an organ counts as a function only if the ancestors of the organ’s owner were naturally selected partly because they had similar organs that performed the same activity.
However, this construal is not satisfying intuitively. To say that x is present because it has a function is normally taken to mean, roughly, that x is present because it is supposed to do something useful. This looks like the right sort of answer to a “What for?” question. Unfortunately, this normal interpretation immediately makes the explanation scientifically problematic, because the claim that x is supposed to do something useful appears to be normative and non-objective.
One possible ground for such a claim is that a designer meant x to do F…If the designer is held to be Nature, the claim involves a metaphorical personification. Dennett (1987) argued that discerning natural functions always involves tacitly conceiving Nature as a designer.
 “Teleological Explanation,” W. H. Newton-Smith, ed. A Companion to the Philosophy of Science (Blackwell 2001), 492-93.