In May 2011, the McMaster University Program in Legal Philosophy sponsored a conference on the theme The Nature of Law: Contemporary Perspectives (<tnl. mcmaster.ca>).* Over 100 specialists and leading figures in legal theory participated. The conference featured seven keynote addresses and twenty-one presentations culled from a pool of almost eighty submissions. As the number and quality of conference submissions and participants attest, the nature of law is a popular and vibrant area of inquiry. With the aim of advancing inquiry in this area, we have compiled a number of papers from the conference and three commissioned works on the conference theme. Part I of the volume comprises five chapters that advance traditional debates within analytic jurisprudence. In the first contribution to this part, Mark Murphy argues that natural law legal theory does not respect the boundaries of one of these debates—the debate that pits positivists who hold that social facts alone are the fundamental determinants of what norms count as law in any given legal system against non-positivists who hold that moral considerations necessarily couple with social facts to determine the laws of any such system. Hartian legal theory is the leading contemporary exemplar of positivist legal theory. On the Hartian view, the laws of a legal system are those specified by that system’s rule of recognition. The rule of recognition is the standard that the system’s officials converge in following and that specifies the system’s criteria of legal validity. As such, the rule of recognition depends on social fact, for it is a matter of social fact what criteria of legal validity a system’s officials converge in following. Hence, on the Hartian view, legal systems’ laws are fundamentally determined by social fact, i.e. the relevant system’s rule of recognition.