IN the 1992 U.S. Supreme Court case of Planned Parenthood v.  Casey, the majority interrupted its lengthy discussion of abortion jurisprudence with a bold pronouncement: “At the heart of liberty is the right to define one’s own concept of existence, of meaning, of the universe, and of the mystery of human life.” This audacious postmodernist creed seems out of place in a dry judicial opinion. But in his new book Natural Bights and the Bight to Choose, † Hadley Arkes argues that it in fact offers the key to understanding what’s at stake in the abortion debate. For good and for bad, it is also central to Arkes’s entire approach to the abortion question.