Table Of ContentTHE SOURCES OF INTERNATIONAL LAW
ii
The Sources
of International Law
Second Edition
HUGH THIRLWAY
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Preface
For a new edition of a work of this kind to be called for only five years after
the original was published is an indication of the extent to which its subject-
matter is still in continuous development. This is not to say that new sources
are being discovered or devised: one of the contentions advanced in this book
is that in this respect international law is fully developed, that what may ap-
pear to be a new source of law will turn out, on inspection, to be a variant,
or a derivation, of one of those classically recognized. But that does not mean
that the law itself deriving from those sources is static; it is continually called
upon to apply to new questions, or to mould itself to new requirements, and
these may be revelatory of particular aspects of sources- theory.
It is one recognized source in particular that goes on requiring or at-
tracting the attention of international scholars and judges: customary inter-
national law, to use the form of reference preferred by the International Law
Commission; also referred to in the past simply as ‘custom’. Despite a very
visible presence in international relations of major multilateral treaties and
other documentary material (some in effect codificatory, e.g., ILC reports
and conclusions), custom continues to attract the most attention, and this for
two reasons. First, being a more flexible concept and process than treaty law,
it is continually being revised and re- examined in practice and, significantly,
in judicial decisions; and secondly, it is the most fertile field for the enunci-
ation of new theories of how it operates, or how it ought to operate. These,
or some of them at least, whether one agrees with them or not, demand to be
noticed, if not necessarily discussed, if a study of sources is to make a claim
to completeness.
In this context, a development since the first publication of this work that
required to be taken into account was the completion, by the International
Law Commission, of its study of the Identification of Customary International
Law, with the adoption of a restatement in the form of sixteen conclusions,
with detailed annotations (see Chapter III). However, in addition, new
treaties have been adopted, and new judicial and arbitral decisions given, and
the nature of the sources of law is such that they have been in background of
most of these, avowedly or not. The work of preparation of this new edition
has not lacked material.
As in the case of its predecessor, many of the ideas presented in this edition
have benefitted from discussion with my good friend and former colleague
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Dr Cristina Hoss, Legal Officer, ICJ. Once again also, I owe a great debt of
gratitude to the Library of the International Court of Justice, now directed
by Mr Cyril Emery, Librarian (the successor to Juliana Rangel, whose assist-
ance I acknowledged in the previous edition), for help and support in my re-
searches. I wish particularly to thank Mr Artur Brodowicz, Deputy Librarian;
not only was he able to find everything I asked for, with great promptitude,
but in other respects he went out of his way to be helpful. The staff of the
Oxford University Press have guided the text from draft to publication with
their usual courtesy and efficiency.
The Hague, 27 January 2019
Hugh Thirlway
Table of Contents
Glossary of Latin Phrases xi
Table of Cases xiii
Table of Legislation xix
I. The Nature of International Law and the Concept of Sources 1
1. Introduction 1
2. Formal and material sources 6
3. Enumeration of the recognized formal sources 8
4. Nature and operation of the sources 12
5. Whose law? States and non-S tate actors 20
6. Are there additional formal sources, not in Article 38? 24
6(a) Unilateral acts 25
6(b) Decisions of international organizations 26
6(c) Agreements between States and international enterprises 28
6(d) Other proposals 29
7. Religious law as a rival or additional source 31
8. Is the theory of sources still sufficient? 34
II. Treaties and Conventions as a Source of Law 37
1. Pacta sunt servanda 37
2. The limits of treaty- law: jus cogens and the relative effect
of treaties 41
3. Commitment to the treaty- obligations 44
4. Unilateral acts as inchoate treaties? 51
III. Custom as a Source of International Law 60
1. Introduction 60
2. Constituent elements of custom 64
2(a) Two elements or one? 64
2(b) State practice 71
2(c) The opinio juris 84
2(d) The role of international organizations 92
3. Changes in customary law 94
4. The relevance of ethical and similar principles
to customary law 97
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5. The extent of application of a rule of customary
international law 99
5(a) General customary law and the ‘persistent objector’ 99
5(b) Particular customary law 103
IV. General Principles of Law as a Source of Law 106
1. What are the ‘general principles of law’? 106
2. The role of equity 119
3. General principles of law and non liquet 125
V. The Subsidiary Sources 131
1. Introduction 131
2. Judicial decisions 134
2(a) International tribunals 134
2(b) Municipal courts 140
3. The teachings of publicists 143
VI. Interaction or Hierarchy between Sources 147
1. Simultaneous and identical obligations under treaty
and under customary law 148
2. The ‘hierarchy of sources’ 152
VII. Specialities: jus cogens, Obligations erga omnes, Soft Law 162
1. Superior norms and their sources: jus cogens
and obligations erga omnes 162
1(a) The source or sources of obligations erga omnes 166
1(b) The source or sources of norms of jus cogens 173
2. Soft law 186
VIII. Subsystems of International Law 195
1. ‘Self- contained regimes’ and their limits 195
2. Human rights law 197
2(a) Human rights law under treaty and as custom 198
2(b) Human rights as deriving from general principles 203
2(c) Human rights and Islam 204
3. Humanitarian law 208
3(a) Treaties and conventions 208
3(b) Customary law or an independent source of law? 208
4. WTO, ICSID: trade and investment law dispute settlement 213
5. International environmental issues 218
6. International criminal law 220
Table of Contents ix
IX. Some Alternative Approaches 223
1. Inadequacy or irrelevance of recognized sources 230
2. The role of ethical principles 232
3. The insufficiencies of the theory of international
customary law 235
X. A Brief Note in Conclusion 238
Index 241
