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Several fundamental problems confront those seeking to find the sources of international law. First and foremost, at the conceptual level, there is no constitutional ‘machinery for the creation of rules of international law’ so that the notion of ‘formal sources’ is misleading (Crawford  2012, 20). Additionally there is the phenomenon of ‘fragmentation’ of international law (see e.g. Koskenniemi 2007; Shaw 2014  46) and the whole question of non-state actors (e.g. Roberts and Sivakumaran in Evans 2018 p 89 - 118).

What we can search for is evidence of ‘general consent of states [that] creates rules of general application’; sources that may provide such evidence are, for example, decisions of the International Court of Justice (ICJ), United Nations General Assembly resolutions and various ‘law-making’ multilateral treaties (Crawford 2012, 20). But these ‘sources and evidences’ are extensive, diffuse and decentralized. Even locating them is a challenge.

This account of sources conforms to the categories of Article 38(1) of the ICJ’s Statute, but adds some additional materials, not specifically identified in the Statute. These are: Section 5 which deals with ‘Other Sources’, such as UN Resolutions and ‘soft law’; and Section 6, listing ‘Guides, Encyclopaedias and Digests’ which are useful as starting points for searches on particular problems or topics.

Particular topics (e.g. human rights, environmental law, and specialised aspects therein) are sometimes viewed as generating independent bodies of law, but they are still part of the general corpus of public international law. Thus materials relevant to such topics are nested within several of the main categories here outlined. Researchers will need to adopt systematic strategies for tracking down required items: a good guide to formulating plans for such cross-referencing can be found in Hoffman and Rumsey (2012, chapter 9).

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