WE are now in the second decade of what has been called the “biological revolution”: the development of new technologies for the control of biological processes. It is well known that these new technologies—such as genetic screening and abortion, laboratory fertilization and embryo culture, gene splicing, and cloning— raise unprecedented and seemingly insoluble ethical dilemmas. Yet the biological revolution also poses a more serious if less obvious challenge: It forces us to reconsider our traditional notions of morality itself and of man’s place in the world. This challenge is the subject of Leon Kass’s Toward a More Natural Science, which aims “to search out the human significance of the presently new biology, and to search for a yet newer and richer biology that will do justice to matters of human significance.” Kass, who teaches in the College and in the Committee on Social Thought at the University of Chicago, was trained as a physician and a molecular biologist, and for the past fifteen years has been one of the most perceptive analysts of the human consequences of the new biology. Toward a More Natural Science offers a selection of thirteen essays written during this period, three of which originally appeared in The Public Interest.