Soft law in treaty interpretation and the determination of customary rules
This thesis examines how instruments without binding force, known as soft law, can influence both the interpretation of treaties and the determination of customary international law. It asks how texts that are not meant to be legally binding can nonetheless acquire legal significance.
Drawing on the case law of the Permanent Court of International Justice and the International Court of Justice, as well as on the work of the International Law Commission, the study explains the different functions that soft law performs within the international legal order. Although such instruments are not binding, they often guide legal reasoning by clarifying meaning, recording agreement, or reflecting broader developments.
Soft law can assist, at the time a treaty is concluded, in determining the ordinary meaning of terms, identifying its purpose and object, and placing it within the context of existing rules of international law. Soft law adopted later can shape an evolving interpretation, help establish tacit agreement among the parties, and in some cases contribute to informal changes in how a treaty operates.
Soft law also plays a role in customary international law. It can inspire state practice, express the belief that certain conduct is legally required, or clarify the content of customary rules. Its influence appears mainly where soft law reflects or helps to build agreement. This unequal distribution of influence highlights the consensual nature of international law, in which rules gain meaning through acceptance and shared understanding rather than through imposition.