The term teleology locates a series of connected philosophical questions. If we grant that there is such a thing as purposive or goal-directed activity (as we must, since, for example, a political campaign aimed at victory represents a clear, uncontroversial case), we may ask the following questions: (1) By what criteria do we identify purposive activity? (2) What is the nature of the systems that exhibit purposive activity? (3) Does the nature of purposive activity require us to employ special concepts or special patterns of description and explanation that are not needed in an account of nonpurposive activity? And if we grant that there are objects and processes which perform functions (again, as we must, since no one would deny, for instance, that the human kidney performs the function of excretion), we may ask: (4) By what criteria do we identify functions? (5) What is the nature of the systems that exhibit functional...