This book revisits the theory of the sources of international law from the perspective of formalism. It critically analyzes the virtues of formalism, construed as a theory of law ascertainment, as a means of distinguishing between law and non-law. The theory of formalism is re-evaluated against the backdrop of the growing acceptance by international legal theorists of the blurring of the lines between law and non-law. At the same time, the book acknowledges that much international normative activity nowadays takes place outside the ambit of traditional international law and that only a...