Re-engaging with the Pure Theory of Law developed by Hans Kelsen and the other members of the Viennese School of Jurisprudence, this book looks at the causes and manifestations of uncertainty in international law. It considers both epistemological uncertainty as to whether we can accurately perceive norms in international law, and ontological problems which occur inter alia where two or more norms conflict. The book looks at these issues of uncertainty in relation to the foundational doctrines of public international law, including the law of self-defence under the United Nations Charter,...