Exam no. 423227 [600387]

Exam no. 423227

Philosophy and International Law
Oppenheim’s view on the science of international law

Lassa Francis Lawrence Oppenheim successfully set the basis of what we know today
as international law. He worked on the transformation of international law into a science. His
treaty is considered “by common consent the outstanding and most frequently employed
systematic treatise on the subject in the English -speaking countries”.1
According to Oppenheim, t he method of the science of international law is the positive
method. Oppenheim summarises the positive method, explaining that it is “applied by the
science of law in general, and it demands that whatever the aims and ends of a worker and
researcher may b e, he must start from the existing recognized rules of international law as they
are to be found in the customary practice of the states or in law -making conventions.”2 Being a
positivist, Oppenheim considers that the science of international law is based on treaties and
customs. He does not recognise the law of nature, as do thinkers from the other schools: the
schools of the Grotians and the Naturalists.
The positive method is tightly linked to turning international law into a science in
Oppenheim’s view. First of all, positivists consider that the science of international law is an
incomplete field, with much work waiting to be done, work with significant implications: “the
science of international law has a great future to look forward to. Hundreds of ha nds are wanted
to enable a future generation to start codification. […] The all -powerful force of good which
pushes mankind forward through the depths of history will in time unite all nations under the
firm roof of a universally recognized and precisely c odified law ”.3 On the other hand, t he

1 Rovira M. (2013) The project of pozitivism in international law, Oxford: Oxford University Press, p. 43;
2 Oppenheim, L., The Science of International Law: Its Task and Method , The American Journal of International
Law, Vol II, No. 2, p . 333 ;
3 Ibidem , p. 356 ;

Exam no. 423227

Grotians and the Naturalists consider that there are no gaps in the science of the international
law, because a whole system is provided by the law of nature.
Secondly, there can be only one good method of the science of international law,
because otherwise there are contradictory answers to the same questions, which is not very
scientific. Oppenheim considers absolutely necessary to compare the three existing methods in
order to choose the best one. The fact that contributors to the science of international law often
come to very different results is because of the use of different methods. In Oppenheim’s words,
“how can two men come to the same results, if the one abstra cts the rules of international law
from the actual practice of the states, and the other from what he considers to be reasonable, just,
and adequate?”.4 Oppenheim rules out the Grotian method and the naturalistic method, one of the
reasons being the genera l disagreement between professors of the law of nature, which “neither
agree in scope, nor in contents, nor in definitions, nor in the very number and contents of the
rules which they profess to draw from reason”.5
Oppenheim’s argument in favour of the pos itive method has normative, ontological and
epistemolo gical aspects. In his argument, Oppenheim explains that a worker or a researcher
using the positive method in the science of international law has to begin from the existing rules.
He stresses the fact that in other domains of law (such as law of contract, tort, real property)
nobody begins from the law of nature, and neither should they in the science of international law.
On the contrary, Oppenheim explains that customary and conventional rules should be the
foundation to build on in the science of international law. Conventional rules are usually easier to
be understood, because they are written and they often have a clear message. However, this is
not always the case due to several reasons, therefore they need interpretation.
Rules that are more difficult to interpret are those based on custom. The rather large
proportion of custom -based rules in comparison to written rules in international law led to many
differences in textbooks. Oppenheim criticize s these works, some of which were created by

4 Oppenheim, op. cit., p. 327 ;
5 Ibidem, p. 329;

Exam no. 423227
authors that did not draw the proper line between usage and custom and that sometimes had the
tendency to fill the gaps instead of recognising them as such. Using the positive method in the
science of internatio nal law means that “no rule must be formulated which cannot be proved to
be the outcome of international custom or of a law -making treaty”.6
An important aspect of Oppenheim’s argument for the positive method is the
ontological one. He argues against the idea that international law is not real law (and therefore
the science of international law loses its object), proving that this is not the ca se. Oppenheim
notices that those who think this about international law tend to wrongly define law, as the “body
of rules imposed by a sovereign upon his subjects”.7 He explains international law through the
metaphor as a tree with branches: international law is a branch like others (municipal law,
constitutional law) in the “tree” of law.
In order to stress the superiority of the positive method, Oppenheim uses an
epistemological aspect to his approach. His argument begins as though he was still comparing
the methods based on natural law and the positive method: “The decision as to the right method
depends on two preliminary questions: first, the question of the existence of a natural law a part
of which a natural international law could be; and, secondly, the question whether there can be a
law at all between sovereign states, and therefore whether international law is or is not real
law”.8
Answering to the two questions with solid arguments and evidence (some of which we
already mentioned), Oppenheim sho ws that the law of nature is an old, obsolete method and that
the science of international law can only be based on the positive method. Not only this, but the
law of nature is harshly criticized, as it “weakens the eyes of those who profess it”9 and it
“prevents the proper criticism of the existing positive law”.10 However, this does not mean that
Oppenheim does not acknowledge the important contribution of Grotius, but he considers that he

6 Ibidem, p. 334 ;
7 Oppenheim, op. cit., p. 327 ;
8 Ibidem , pp. 327 -328
9 Ibidem , p. 329
10 Ibidem ;

Exam no. 423227

belonged to a certain age, therefore he could not have thought in o ther terms other than the law
of nature (Grotius was “a child of his time and therefore a product of his age”11).
Moreover, Oppenheim criticized the “phantasms” which were argued by some to be
scientific. He considered that the term science implies knowled ge, therefore those who claim that
have a scientific method should “stick to the facts, and not hunt phantasms”.12
Oppenheim’s conclusion is that the law of nature is unreliable, obsolete and is not fit as
a method for the science of international law. The only good alternative is the positive method.

Bibliography

OPPENHEIM, L., The Science of International Law: Its Task and Method , The American
Journal of International Law, Vol II, N o. 2, pp. 313 – 356
ROVIRA M. (2013) The project of pozitivism in international law, Oxford: Oxford University
Press

11 Ibidem, p. 328
12 Ibidem;