INTERNATIONAL LAW, Sixth edition

112 international law
As has already been seen, the decisions of municipal courts 185 may
provide evidence of the existence of a customary rule. They may also
constitute evidence of the actual practice of states which, while not a
description of the law as it has been held to apply, nevertheless affords
examples of how states actually behave, in other words the essence of the
material act which is so necessary in establishing a rule of customary law. 186
British and American writers, in particular, tend to refer fairly extensively
to decisions of national courts.
One may, finally, also point to decisions by the highest courts of federal
states, like Switzerland and the United States, in their resolution of
conflicts between the component units of such countries, as relevant to
the development of international law rules in such fields as boundary disputes.
A boundary disagreement between two US states which is settled
by the Supreme Court is in many ways analogous to the International
Court of Justice considering a frontier dispute between two independent
states, and as such provides valuable material for international law. 187
Writers 188
Article 38 includes as a subsidiary means for the determination of rules
of law, ‘the teachings of the most highly qualified publicists of the various
nations’.
Historically, of course, the influence of academic writers on the development
of international law has been marked. In the heyday of Natural
Law it was analyses and juristic opinions that were crucial, while the role
of state practice and court decisions was of less value. Writers such as
Gentili, Grotius, Pufendorf, Bynkershoek and Vattel were the supreme
authorities of the sixteenth to eighteenth centuries and determined the
scope, form and content of international law. 189
p. 14, with regard to the test for state responsibility in respect of paramilitary units. The
International Court indeed reaffirmed its approach in the Genocide Convention (Bosnia
v. Serbia) case, ICJ Reports, 2007, paras. 402 ff.
185 See e.g. Thirty Hogsheads of Sugar, Bentzon v. Boyle 9 Cranch 191 (1815); the Paquete
Habana 175 US 677 (1900) and the Scotia 14 Wallace 170 (1871). See also the Lotus case,
PCIJ, Series A, No. 10, 1927, p. 18; 4 AD, p. 153. For further examples in the fields of state
and diplomatic immunities particularly, see below, chapter 13.
186 See e.g. Congo v. Belgium, ICJ Reports, 2002, pp. 3, 24; 128 ILR, pp. 60, 80.
187 See e.g. Vermont v. New Hampshire 289 US 593 (1933) and Iowa v. Illinois 147 US 1 (1893).
188 See e.g. Parry, British Digest, pp. 103–5 and Lauterpacht, Development of International Law,
pp. 23–5. See also R. Y. Jennings, ‘International Lawyers and the Progressive Development
of International Law’ in Makerczyk, Theory of International Law at the Threshold of the
21st Century, 1996, p. 325, and Pellet, ‘Article 38’, p. 790.
189 Seeabove,chapter1.