George William Van Cleve's A Slaveholders' Union includes an interesting discussion of English law concerning slavery leading up to Somersett's Case (1772), the dramatic change in the law that that decision represented, and the discussion (and in some quarters concern) that it prompted in the North American colonies. See also George Van Cleve, Somerset's Case and its Antecedents in Imperial Perspective.
Until the 1760s, the English Crown generally supported and the institution of slavery and “sought to have English law treat slaves as uniform 'imperial' property, since British investors and creditors thought that they needed predictable legal rules to support what they perceived as risky investments in the slave trade and colonial plantations.” This approach was exemplified by the so-called Yorke-Talbot opinion of 1729, in which the Crown's two chief law officers issued an opinion stating that colonial slaves remained property when brought to England:
We are of opinion, that a slave coming from the West-Indies to Great-Britain or Ireland, with or without his master, doth not become free, and that his master's property or right in him is not thereby determined or varied; and that baptism doth not bestow freedom on him, or make any alteration in his temporal condition in these kingdoms. We are also of opinion, that his master may legally compel him to return again to the plantations.
Three years later, “[t]he goal of enforcing uniformity” with respect to slave property led Parliament to pass An Act for the More Easy Recovery of Debts in His Majesty's Plantations and Colonies in America of 1732, “a 'sweeping' statute that guaranteed uniform imperial treatment of slave property for debt-recovery purposes in England and its colonies, overriding all contrary colonial laws.” The Act, “the legislative analogue of the” Yorke-Talbot opinion, provided that
“Negroes” (slaves) in the colonies were classified as property for purposes of debtor-creditor relations. Creditors throughout the empire were given a broad range of remedies to protect their interests in such property. . . . The act thus created a hybrid form of property valid throughout the empire. It effectively overruled a House of Lords decision that had respected colonial law in this area.
A Harvard Law Review article describes the Act as follows:
In 1732, to advance the economic interests of English merchants, Parliament enacted a sweeping statute, the Act for the More Easy Recovery of Debts in His Majesty’s Plantations and Colonies in America, which required that real property, houses, and slaves be treated as legally equivalent to chattel property for the purpose of satisfying debts in all of the British colonies in America and the West Indies. This statute substantially dismantled the legal framework of the English inheritance system by giving unsecured creditors priority to a deceased’s land over heirs. The Act also required that the courts hold auctions to sell both slaves and real property to satisfy debts in most colonies. More broadly, this legal transformation likely led to greater commodification of real property, the expansion of slavery, and more capital for economic development. American landholders, however, were subjected to greater financial risk than would have been the case in the absence of the Act.
There the law remained through the 1760s, but by that point opinions concerning slavery were changing among some in England. In the latter half of the 1760's, the great English legal commentator William Blackstone analyzed the slavery question in his landmark treatise Commentaries on the Laws of England (1765-1769). Although Blackstone was a quintessential establishment figure and no rebel, he branded slavery “an arbitrary and despotic power” inconsistent with English law (Volume 1, pp. 122-123, emphasis added):
THE idea and practice of this political or civil liberty flourish in their highest vigour in these kingdoms, where it falls little short of perfection, and can only be lost or destroyed by the folly or demerits of it's owner: the legislature, and of course the laws of England, being peculiarly adapted to the preservation of this inestimable blessing even in the meanest subject. Very different from the modern constitutions of other states, on the continent of Europe, and from the genius of the imperial law; which in general are calculated to vest an arbitrary and despotic power of controlling the actions of the subject in the prince, or in a few grandees. And this spirit of liberty is so deeply implanted in our constitution, and rooted even in our very soil, that a slave or a negro, the moment he lands in England, falls under the protection of the laws, and with regard to all natural rights becomes eo instanti [at that moment] a freeman.
This discontent over the legal treatment of slavery came to a head in Lord Mansfield's ruling in Somersett's Case in 1772, which he issued although he acknowledged its destabilizing effect (“We feel the force of the inconveniences and consequences that will follow the decision of this question”).
The key to the decision was Lord Mansfield's underlying conclusion that slavery was a status or relationship conferred by law, not an objective reality or thing (a horse is always going to be a horse, no matter what the law says). Moreover, status was location and jurisdiction dependent. One man might own another under the laws of one country. But if master and servant relocated to another country, the laws of that country applied, and if those laws did not recognize or authorize the status or relationship of slavery, then that condition no longer applied:
So high an act of dominion [as holding another in slavery] must derive its authority, if any such it has, from the law of the kingdom where executed. A foreigner cannot be imprisoned here on the authority of any law existing in his own country: the power of a master over his servant is different in all countries, more or less limited or extensive; the exercise of it therefore must always be regulated by the laws of the place where exercised.
Slavery, then, could not exist unless authorized by “positive law” of the jurisdiction in which the dominion of slavery was exercised. Although Lord Mansfield did not spell out precisely what “positive law” consisted of, he did specify that it excluded judge-made law exemplified in England by the Common Law, by which judges fashioned non-statutory rules of responsibility and liability in a variety of contexts (for example, what degree of negligence, if any, was necessary to hold a defendant liable for damages in a personal injury action, or the rules that determined whether a binding contract existed). Slavery was “so odious” that something more – presumably a statute of parliament or perhaps “immemorial usage” – was required to constitute the “positive law” required to create it:
The state of slavery is of such a nature, that it is incapable of now being introduced by Courts of Justice upon mere reasoning or inferences from any principles, natural or political; it must take its rise from positive law; the origin of it can in no country or age be traced back to any other source: immemorial usage preserves the memory of positive law long after all traces of the occasion; reason, authority, and time of its introduction are lost; and in a case so odious as the condition of slaves must be taken strictly.
From there, decision in favor of the slave (or non-slave) followed rapidly. Because England had no such non-judge-made positive law, “the man must be discharged”:
The power claimed by this return [i.e., the master's response to the petition of habeas corpus, in which he asserted that he was entitled to hold the petitioner as a slave] was never in use here; no master ever was allowed here to take a slave by force to be sold abroad because he had deserted from his service, or for any other reason whatever; we cannot say the cause set forth by this return is allowed or approved of by the laws of this kingdom, therefore the man must be discharged.