Biology refers both to the systematic investigation of living things, and to the body of knowledge that is the product of that investigation. Throughout biology's history, however, some important questions debated by biologists have not been so much about the organisms being studied, but about the nature of life, the proper way to investigate it and the form biological knowledge should take. When inquiry shifts from questions about living things to questions about proper and improper ways of asking, or answering, or adjudicating, such questions, it shifts to a philosophical level. One need not, of course, be trained in a department of philosophy to contribute to such an inquiry. Indeed many of the most significant contributions to the subject have been made by people trained in the sciences. Nevertheless such contributions are to the subject designated as philosophy of biology.

One assumption implicit in...