To interpret ‘slavery’ in modern international and domestic law we do well to look at its history in law and philosophy. A convenient starting point is Roman law, because that was the first system that regulated slavery in detail and at the same time conceded that it was open to objection as ‘contrary to nature’. By nature, legal texts affirm, we are free and equal members of a community related by birth. According to Ulpian, writing in the early years of the third century AD, though in civil law slaves are treated as nullities, by natural law this is not the case, because by natural law all people are equal.1 He also asserts that by natural law all people were born free. There was at first no freeing of slaves (manumission), for there was no slavery.2 Later, however, by the law of peoples (ius gentium) slavery broke in.3 The phrase ‘law of peoples’ refers to the general custom of cities and political entities in the ancient world. I shall call it ‘general political practice’. The introduction of slavery, says Ulpian, led to the benefit of manumission.
Much obviously turns on how we are to understand ‘nature’. In what sense was it thought to bind us? It did so at least in regard to what we would now call our inescapable genetic and cultural endowment, our inbuilt drive to survive, propagate, socialise, and to preserve our families and communities. Could it also be regarded as obligatory, a form of law? Perhaps, at least to the extent that we are bound to respect these drives, and the innate capacities of reason and communication that help to make their exercise effective. In any case a number of lawyers of Ulpian’s time said much the same. To Florentinus slavery is a general political practice by which a person is subjected to the ownership of another contrary to nature.4 To Tryphoninus freedom is part of natural law and the ownership of slaves5 was introduced by general political practice.6 Marcianus says of a person born a slave who has received from the emperor a grant of free birth that he was not in fact born free, in the condition in which all men initially lived.7 Nor are these opinions mere theory. Ulpian records that many legal rulings in favour of liberty are made contrary to strict law.8
If, then, human beings are, or were originally, born free and equal, why was slavery a recognised institution? To advocate its abolition in the Roman world would, given the interests of slave owners, have been futile. But thoughtful people argued that its extent should be restricted. When the law could be interpreted in a way that favoured freedom, freedom should be preferred: favor libertatis. According to Hans Ankum there are 31 texts between the legal author Iavolenus in the early second century AD and Ulpian in the third that use this phrase as an argument to interpret the law in favour of freedom.9 The lawyers who use the phrase were not attacking slavery in principle but wished to limit its application. This restrictive view, like the argument adopted by the northern states of the US against the extension of slave territories in the Missouri compromise of 1820, or that in the UK about the same period for abolishing the slave trade but leaving slavery itself untouched, took root slowly. But by the Severan age, in the late second and early third centuries AD, the limiting view had come to prevail in Roman legal circles. In the cosmopolitan atmosphere of that period citizenship was extended to all free people in the empire. Slaves were excluded from the general grant of freedom but a number of legal authors, we saw, endorsed the view that they, like other people, are by nature free, or born free. To say that they are born free is not to deny that babies, though they are aware of their own constitution and find it congenial,10 depend on adults. The sense in which they are born free is that they possess from birth the potential to become free people. They are born, like other animals, with the instinct to preserve and propagate themselves11 and to understand what is salutary or life-threatening.12 Humans are further born with the capacity to learn to reason, to choose between alternatives and to decide what to accept or reject. Slavery is from this point of view an institution that inhibits these natural capacities in those subject to it. It is not innate in human beings.
The background to this way of thinking is that we are by nature citizens of a universal community, a cosmopolis. Since citizens are by definition free, a citizen of the universal community cannot be a slave. We are therefore equal in certain respects: as citizens of a (worldwide) social community, and as possessing the capacity for reason. This common membership, sociability and reasoning power is part of human nature.13 As Florentinus said, nature made us cognates, members as it were of the same family.14 In part these opinions reflect a long-standing Roman political tradition and in part a theory derived from Greek, particularly Stoic, philosophy.15 Lawyers are not professional adherents of this or that school of philosophy, but they are entitled, in support of their legal opinions, to endorse some elements of a philosophical tradition. Intellectually, they can scavenge.
Some writers deny the influence of Stoic philosophy on the Roman lawyers mentioned, because the Stoics no more advocated the abolition of slavery than did other ancient thinkers. They thought that virtue was the sole good, and that even a slave could be virtuous. Indeed no virtuous person was really a slave, since it is our emotions and passions that enslave us.16 Despite these mistaken or self-indulgent views about virtue, a number of Stoics and like thinkers endorsed the idea of a universal human community.17 As Cicero puts it:
[T]hey [Stoics] think it important to understand that nature creates in parents love for their children; and from this source we derive the general sociability of the human race…. Even among animals nature’s power can be observed; when we see the effort that they spend on giving birth and rearing their young, we seem to be listening to the voice of nature itself…. Hence it follows that mutual attraction among humans is also something natural. The mere fact of their common humanity requires one man not to regard another as alien.18
Moreover, in the Stoic view, living creatures should live according to their respective natures and constitutions,19