This study of legal validity is an expanded and thoroughly revised version of my B.Phil. thesis in philosophy at Oxford University in 1969. I am grateful to Professor R. M. Hare, Dr. P. M. Hacker, and Mr. L. J. Cohen for their patient criticism of earlier drafts, and to Professor Donald H. Regan for several suggestions at a later stage. I owe a much larger debt to Professor H. L. A. Hart for his detailed comments on the completed thesis. His help has been especially gener ous in light of the fact that I have so often disagreed with him. It should not be assumed that those from whose advice I have benefited share the views expressed in this essay. I am responsible for any mistakes it may contain. In the footnotes I have used the following abbreviations: CL - Hart, The Concept of Law (1961) GT - Kelsen, General Theory of Law and State (1945) PT - Kelsen, Pure Theory of Law (1967) LJ - Ross, On Law and Justice (1958).