International Judicial Monitor
Published by the International judicial Academy of the International Law Institute, Washington, D.C.
with circulation assistance from the American Society of International Law

Winter 2018 Issue

Historic Moments in International Law

On Nature, Nations and an Odious Trade

Stephen C. NeffBy: Stephen C. Neff, Reader-in- Law, Public International Law, University of Edinburgh Law School

International law, for many centuries, was referred to by the now antiquarian-sounding label of the “law of nature and nations.”  (Indeed, at my own institution, the University of Edinburgh, there is a professorial chair of that title, established in 1707 and currently held by Neil Walker).  It should not be thought, however, these two labels refer to the same thing.  In brief, the law of nature refers to eternal, immutable, intransgressible, universal principles.  The law of nations, in contrast, is more down to earth.  It refers to the aggregate of customs and practices devised by the various states of the world in the course of their day-to-day affairs.

There is no more vivid a demonstration of the distinction between these two kinds of law than on the on the emotive subject of slavery.  As far back as 3rd Century A.D., the famous Roman jurist Ulpian held that there is no such thing as slavery in the law of nature – a statement preserved in the Digest of Justinian (compiled in the 6th Century).  Slavery, explained Ulpian, is purely a product of the law of nations, meaning that it is entirely a human artefact, rather than an ineluctable feature of the moral universe.  This subtle distinction between the law of nature and of nations continued to exist into the 19 Century – and, more specifically, to bedevil campaigners against the slave trade.

By the end of the 18th Century, agitation against slave trading had reached significant levels in Europe.  The British Abolition Society, for example, was founded in 1787.  It became apparent, though, that there were two quite distinct legal means by which the anti-slave trade campaign could proceed.  One was to persuade as many states as possible to prohibit slave trading in their national laws.  The first notable step in this direction was taken by Denmark in 1792.  The United States and Britain both prohibited the slave trade by legislation in 1807.  As more and more countries followed suit, a case could gradually be built up for holding slave trading to be unlawful according to the law of nations.

The other possible legal strategy was more drastic and intrusive.  It was to hold slave trading to be unlawful according to the law of nature.  This could then be argued to have the momentous consequence that slave trading would be a violation of international law even if it continued to be allowed by the national laws of the various countries.  On this view, slave trading would be regarded as, more or less, on a legal par with piracy, as an offence against human society at large.  It would then be possible for states to take action against any and all slave traders on the high seas – regardless of their nationality and regardless of the status of slave trading under the national laws of the traffickers’ states.  This would later be given the label of universal criminal jurisdiction.

Politicians being as they are (to put as politely as possible), the safer and more gradual course of state-by-state abolition was the solution that was envisaged.  This achieved its classic exposition at the Congress of Vienna in 1815, with the adoption by the assembled powers of a grand-sounding Declaration on the Abolition of the Slave Trade.  This Declaration was, in reality, the very soul of caution.  It carefully noted the agreement of the powers that the abolition of the slave trade was “a measure particularly worthy of their attention.”  At the same time, however, the signatory powers noted that it was not possible to ignore “the interests, the habits, and even the prejudices of their subjects” on the matter.  Consequently, it was not possible to determine the time period within it would be “most advisable” for any given power actually to prohibit the trade – a matter that was then left for future negotiations.  (Enthusiasts of “soft law,” incidentally, could usefully cite this Declaration as a very early illustration of that phenomenon.)

If the politicians were wary of proceeding too far and too fast, there were signs that judges were prepared to be bolder – by holding slave trading to be, here and now, a violation of international law against which any and all countries could take action. An early blow in this direction was struck by British courts in the case of the Amedie, in 1810.  Sir William Grant, with considerable eloquence, flatly pronounced the slave trade to be “altogether illegal.”  He stopped short, though, of fully equating slave trading with piracy, by conceding that British ships could capture slave traders on the high seas only if the traders’ state of nationality prohibited the practice.  Grant did insist, however, that the burden of proof that the trade was allowed by the flag state rested on the trader.  And in the particular case, he was clearly pleased to note that that burden had not been discharged.

The anti-slave trade cause, however, suffered a sharp setback shortly afterwards, in the British admiralty case of the Louis in 1817.  The judge was the renowned admiralty lawyer Sir William Scott (later enabled as Lord Stowell).  Scott carefully considered the question of whether or not slave trading was unlawful under international law – and concluded that it was not.  His international law, it is clear, was the law of nations rather than of nature.  International law, Scott explained, arose from the “general and ancient and admitted practice” of states, including the “general tenour of [their] laws and ordinances.”  Looking at this general body of state practice, Scott concluded that the consensus of nations had not evolved to the point of holding the slave trade to be unlawful on the analogy of piracy.  Consequently, there could be no right to visit and search foreign vessels on the high seas even if the flag states in question had abolished slave trading in their national laws.  Visit and search were reserved for the two traditional cases of war and piracy, and extended no further.  If a foreign state took the laudable step of prohibiting the slave trade in its national law, then the enforcement of that law was exclusively the prerogative of that state – with no right of other countries to intermeddle.

A parallel legal battle was soon to be waged in the courts of the United States.  The initial salvo was the case of the Jeune Eugénie, decided in 1821 by Supreme Court Justice Joseph Story, sitting in a circuit-court case.  Contrary to the view of Scott, Story held slave trading to be a violation of international law, decrying it as “a trade abhorrent to the great principles of Christian morality, mercy, and humanity.”  Crucial to this conclusion was an insistence on Story’s part that international law encompassed not only the customs and practices of states, but also “general principles of right and justice.”  Moreover, contended Story, “no [state] practice whatsoever can obliterate the fundamental distinction between right and wrong.”  In effect, then, Story regarded Scott as erring in holding international law to comprise only the law of nations, to the exclusion of the law of nature.  Story expressly regretted that Scott had been prepared “silently to abandon and repudiate” the noble judgment of Grant.

The full bench of the U.S. Supreme Court, however, was shortly to play Scott to Story’s Grant.  This was in the landmark case of the Antelope, decided in 1825.  A U.S. vessel had seized the Antelope, with a cargo of several hundred slaves, which had earlier been taken from three ships off the coast of Africa – one each of Spanish, Portuguese and American nationality.  The Spanish and Portuguese consuls asserted title to the slaves taken from their respective vessels. 

Chief Justice John Marshall, writing for the Court – including a very silent Justice Story -- followed the lead of Scott.  Unlike Scott, however, Marshall noted the distinction between natural law and the law of nations on the subject of slavery.  It could “scarcely be denied,” Marshall conceded, that slave trading was “contrary to the law of nature.”  It was not, however, contrary to the law of nations, i.e., to the practices of states.  Quite the opposite, in fact.  The trade “has been sanctioned in modern times,” noted Marshall, “by the laws of all nations who possess distant colonies.”  It was therefore to be regarded by the law as “a common commercial business which no [third party] could rightfully interrupt.”  Consequently, the lawfulness or unlawfulness of a slave trader’s action must be wholly determined by the national law of the person’s home state.  The fundamental principle of “the perfect equality of nations,” Marshall ruled, precluded any state from lawfully disrupting the trade of another.  In short, there could be no general license (or universal jurisdiction) to interfere with the slave trade unless that trade was contrary to both the law of nature and the law of nations.

It is easy to condemn Marshall’s judgment in the Antelope.  Who nowadays would dream of defending the lawfulness of slave trading?  It is more difficult, however, to be quite so categorical about the underlying view of the nature of international law, on which the judgments of Scott and Marshall were founded.  The law of nature may make all persons free, as Ulpian had proclaimed.  But the fact was that, in the 19th Century, the law of nature was falling into serious disrepute as a source of international law.  Scott and Marshall were clearly in tune with the dominant trend of the time in holding international law to be a product of state practice and the consensus of states, rather than a reflection of eternal, transcendental principles of morality.  (We now call this the positivist view of international law.)

At the same time, though, ideas of natural law have retained a certain stubborn, if vestigial, presence.  Even in the positivist heyday of the 19th Century, some writers kept the natural-law banner waving.  Johann Caspar Bluntschli, the noted Swiss lawyer, for example, contended that treaties concluded in violation of natural law were void – with treaties supporting slavery given as a specific example.  The British writer William Edward Hall took the same view.  There is even a place in modern international law for this way of thinking, in the form of peremptory norms (or jus cogens to those of Latinate bent), which render conflicting treaty provisions void.  There is a certain uneasiness on the part of some – and hearty approval from others – about this idea that certain “master norms” exist which can prevail even over the practices of states.

Be that as it may, the effort to enlist the law of nature and universal jurisdiction in the anti-slave trade cause foundered in the British and American courts.  That left the more cautious approach of conscious state-by-state action as the only viable legal strategy.  The story of the playing-out of that strategy is a fascinating one, which may form the subject of a future column.  For now, it is only necessary to appreciate that the venerable distinction between the laws of nature and of nations is no mere dusty relic of Roman law.  It is part of our legal and moral lives to the present day.

ASIl & International Judicial AcademyInternational Judicial Monitor
© 2018 – The International Judicial Academy
with assistance from the American Society of International Law.

Editor: James G. Apple.
IJM welcomes comments, suggestions, and submissions.
Please contact the IJM editor at