This book is about a fundamental change in American legal thought that took place in the late 19th and early 20th centuries. Before the change, natural law played an important role in our legal system. Lawyers believed that human affairs were governed in significant part by laws of nature, laws that could be discovered in much the same way as the natural laws governing the nonhuman realm. After the change, lawyers believed that all the rules of the legal system are created by humans. This book will explain the functions natural law performed, why lawyers stopped using it, and how those functions came to be performed once natural law was no longer a component of the legal system.