The question: why do sovereign states, with no sovereign above them, feel legally bound? The theories (a guaranteed theory question):
1. Naturalist theory (Grotius, Pufendorf): international law binds because it derives from nature/ reason — a law higher than state will. Critique: whose reason? unverifiable, unenforceable.
2. Positivist theory (Austin, Anzilotti): law = the will/consent of states (treaties express; custom implied consent). Critique: consent cannot explain why new states are bound by old custom, nor the binding force of jus cogens (which overrides consent).
3. Auto-limitation theory (Jellinek): the state binds itself by its own will's self-restriction. Critique: what a state limits, it can un-limit — obligation collapses into discretion.
4. Normative/pure theory (Kelsen): international law as a hierarchy of norms grounded in the Grundnorm (the presupposed basic norm — "states ought to behave as they have customarily behaved"), municipal systems deriving validity from it (primacy-of-international-law theory).
5. Sociological/necessity theory: law reflects the needs of interdependence — states obey because cooperation is indispensable (the functional account behind modern regime theory).
6. Pacta sunt servandum as basis — good faith in agreements kept (the Vienna Convention on the Law of Treaties, Article 26, its charter): the doctrine most practical lawyers give as the true foundation — obligations are honoured because the system's members accept that they must be.