Does international law's effectiveness require a clear distinction between law and non-law? This essay, which reviews Jean d'Aspremont's Formalism and the Sources of International Law, argues the answer is no. Ambiguity about the legal nature of international instruments has important benefits. Clarity in the law may encourage states to do the minimum necessary to comply, while some uncertainty about what the law requires may induce states to take extra efforts to ensure they are in compliance. Ambiguity in the law also promotes dynamic change, an important feature in rapidly developing areas of the law such as international environmental law and human rights. Most importantly, though, soft law — international instruments that have legal consequences but are not unambiguously 'law' — expands the range of instruments available to states when cooperating. Institutionalist theories of international law suggest that a larger menu of international instruments is valuable because it allows states to calibrate the level of their commitments more precisely, thereby expanding their ability to cooperate. Institutional theories, however, have heretofore not explained exactly how states communicate to each other the level of their commitment; that is, they have not explained how states mark an instrument as soft law and whether and how states distinguish between types of soft law commitments. A theory of law-identification based on linguistic norms, such as d'Aspremont proposes, offers a descriptive account of how states might signal levels of legal commitment beyond the dichotomy of 'binding' and 'non-binding' law. A communicative theory of international law — one based on the use of language in international instruments to signal relatively fine-grained variation in the level of commitment — thus would enrich our understanding of what soft law is, and when and how states use it.