230 A Positive Century (1815–1914)of Luxembourg, was a professor at the École libre des sciences politiques in
Paris, with sociology as his principal fi eld. Sorel did diplomatic work at the
French Foreign Offi ce for some six years before becoming a professor, like
his coauthor, at the École libre des sciences politiques. His academic work
was primarily in the fi eld of diplomatic history. He was also a poet and nov-
elist, whose literary polish won him election to the venerable Académie
française. Th eir treatise was published in 1877. Another notable fi gure was
Alphonse Rivier. Originally from Lausanne in Switzerland, he was educated
in Germany and became a professor fi rst at the University of Berne and then
at the University of Brussels.
Th e single most forthright pre sen ta tion of the empirical version of posi-
tivism came from a German who transplanted himself to Britain, Lassa Op-
penheim. Originally from the vicinity of Frankfurt, he taught at the Univer-
sities of Freiberg and Basel in Switzerland— though in criminal law rather
than international law. He moved to Britain in 1895, apparently for health
reasons, becoming a British national fi ve years later. He shift ed his interest to
international law, which he taught fi rst at the newly founded London School
of Economics and then (from 1908– 19) at Cambridge as Westlake’s successor
in the Whewell professorship. His massive work, International Law: A Trea-
tise, fi rst published in 1905– 6, became something of an unoffi cial canonical
summation of international law, at least for British lawyers. In an article
published in the American Journal of International Law in 1908— entitled,
appropriately, “Th e Science of International Law”— Oppenheim set out what
remains the fi nest exposition of the empirical approach to positivism.
Of the distinctive features of this variant of positivism, two should be
singled out for special emphasis. First was a powerful focus on custom, in-
stead of treaties, as the major source of international law. Law was seen— as
in all versions of positivism— as a product of will. For partisans of the em-
pirical viewpoint, this meant the collective will of the international commu-
nity at large. Oppenheim, for example, fl atly held customary law to be, in
essence, the sole basis of international law. Treaties, of course, are legally
binding on the parties to them, but only in the sense that contracts are bind-
ing on private parties in national law. Underlying the whole of treaty law is
the fundamental customary-law rule that treaties must be observed—pacta
sunt servanda. Customary law, in turn, is to be discerned by the close and
impartial study of the actual practice of states in their everyday relations—