This is the penultimate draft of what will be the lead paper in volume 2 of Oxford Studies in Philosophy of Law, due out later this year.  (Volume 1 is here.)  Perry's paper is a major contribution to the literature on authority, and so I hope it will whet the appetite of legal philosophers and those interested in the subject for Volume 2.  Other contributors to volume 2 will include Bruno Celano, R.A. Duff, Matthew Kramer, Barbara Levenbook, and C.L. Ten, among others.