The Chief Justice of the King's Bench, Lord Mansfield, ordered a hearing for the following January. In fact, following an adjournment, it was not until February 1772 that the case was heard. In the meantime, it had attracted a great deal of attention in the press, and members of the public were forthcoming with donations to fund lawyers for both sides of the argument. An activist layman, Granville Sharp, who continually sought test cases against the pretended legal justifications for slavery, was Somersett's real backer, and when the case was heard, no fewer than five advocates appeared for the slave, speaking at three separate hearings between February and May. These lawyers included Francis Hargrave, a young lawyer who made his reputation with this, his first ever case.
On behalf of Somersett it was argued that while colonial laws might permit slavery, neither the common law of England nor any law made by Parliament recognised the existence of slavery, and slavery was therefore illegal. Moreover, English contract law did not allow for any person to enslave himself, nor could any contract be binding without the person's consent. The arguments thus focused on legal details rather than humanitarian principles. When the two lawyers for Charles Steuart put their case, they argued that property was paramount and that it would be dangerous to free all the black people in England. Lord Mansfield, having heard both sides of the argument, retired to make his decision, and reserved judgment for over a month.
Somersett was freed, and his supporters, who included both black and white Londoners, immediately celebrated a great victory. Whilst argument by counsel may have been based primarily on legal technicalities, it is evident that Lord Mansfield himself believed a great moral question had been posed, and that he deliberately avoided answering that question in full, because of its profound political and economic consequences. Lord Mansfield is often misquoted as declaring that "The air of England is too pure for a slave to breathe", but no such words appear in the judgment. Instead, they appear repeatedly in the arguments of counsel for Somersett, who cited a report of a case from 1569, in the reign of Queen Elizabeth I, where
However, it is not actually clear that this was said in the Cartwright case either, and many people believe it was actually a misquote of an excerpt from Chief Justice Holt's judgment in Smith v. Brown (1702) 2 Salk 666, where he is reported to have said:
In 1785, Lord Mansfield himself expressed the view in R v Inhabitants of Thames Ditton that all the Somersett case decided was that a slave could not be forcibly removed from England against his will, which would support the account of his judgment given in The Times letter, and is the strongest argument for a limited scope to the decision. Certainly Mansfield's judgment in the Somersett case does not expressly say that slaves became free when they set foot in England - it is silent as to what their status in England was - although 13 years later in the Thames Ditton case, Lord Mansfield appeared to compare a slave's status to that of 'villein in gross' - i.e. an ancient feudal status of servitude which had not technically been abolished from English Law but which had died out in practice. However, he had pointedly not done so in the Somersett case despite the invitation of Steuart's counsel.
Nevertheless, the Somersett judgment, even if limited to prohibiting the forcible removal of slaves from England, plainly did establish a radical precedent. It went against recent legal authority in both the official opinion of the Attorney-General, Sir Philip Yorke and the Solicitor-General, Mr Talbot in 1729, and the court decision of Sir Philip Yorke, by then Lord Chancellor Hardwicke, in 1749 in the case of Pearne v Lisle. These decisions had stated that slaves were mere items of property (Hardwicke described them as 'like stock on a farm') who were not emancipated either by becoming a Christian or by entry into England, that possession of them could be recovered by the legal action of trover and that their master might legally compel them to leave England with him. However, the 1749 claim relied on the 1729 opinion, which itself quoted no precedents or rationale, and there were other cases with different outcomes before 1772, notably Shanley v Harvey (1763) and R. v Stapylton (1771- also before Lord Mansfield)- for details, see article Slavery at common law.
However, there is an argument that the precedent established by Somersett's case went even further, at least in its interpretation in later cases. In The Slave Grace in 1827 Lord Stowell upheld the decision of the Vice-Admiralty Court in Antigua that a slave who had returned to the colonies from England had not been emancipated by her stay in England. Lord Stowell criticised Lord Mansfield's judgment in the Somersett case, describing it as having reversed the judgment of Lord Hardwicke and establishing that "the owners of slaves had no authority or control over them in England, nor any power of sending them back to the colonies". Lord Stowell went onto say "Thus fell a system which had existed in this country without doubt, and which had been occasionally forced upon its colonies and has continued to this day - that is, above fifty years - without further interruption". This wider reading of Somersett's case appears to be supported by the judgment of Best J in Forbes v Cochrane in 1824 where he describes the Somersett case as entitling a slave in England to discharge and rendering any person attempting to force him back into slavery as guilty of trespass.
Whatever the technical legal ratio decidendi of the case, it was widely understood by the public at large to mean that on English soil at least, no man was a slave.
Certainly, it is clear that Lord Mansfield himself would far rather not have made any decision at all. Although he was an abolitionist, he was extremely concerned by the potential economic trauma which sudden abolition would bring and might well have considered the actual slow phasing out of slavery over the next half century as precisely what he had always advocated. The national economy was at the time based to a great extent upon the slave trade and indeed slavery in the colonies, and a judgment against slavery even in England could begin to undermine that system, as some argued that the law of England should apply on English ships even if not in the Colonies. Moreover, Steuart's Counsel, funded and encouraged by the slave merchants, argued that the consequence of a judgment in Somersett's favour may be to free the slaves in England, said to be 14,000 in number. As Lord Mansfield said in the case report "The setting 14,000 or 15,000 men at once free loose by a solemn opinion is much disagreable in the effects it threatens". As a result, both in and out of court he tried to persuade Steuart to release Somersett himself, and so avoid a decision, as he had done in other cases. Indeed, in the Thames Ditton case in 1785, Lord Mansfield himself sought to limit the impact of the Somersett case, although by that time the political climate in England had become considerably more conservative because of the American and French Revolutions and in 1780 his house had been firebombed by a Protestant mob because of his judgments in support of rights for Catholics, which may well have caused him to be more cautious on politically sensitive issues. Far from being an ardent radical, it appears that Lord Mansfield was forced into making a judicial determination that he had always tried desperately to avoid, but in the end, made the decision as best he thought he could.
However, despite all his concerns and all the potential consequences, Lord Mansfield still freed James Somersett. He did so in the face of the opinion of the Attorney-General and Solicitor-General in 1729, men whom Lord Mansfield himself in the Somersett case described as "two of the greatest men of their own or any times". He could simply have followed those decisions as he would have been legally justified in doing, or he could have tried to free James Somersett on a technicality, but, because both the pro- and anti-slavery lobbies wanted a firm ruling, he did neither. Indeed, as is observed in the Slave Grace case, Lord Mansfield's judgment went far further than it needed to go to free Somersett. He described the system of slavery as 'odious' at a time when the slave trade was at its height and the abolition movement was still in its infancy. The prominence given to the case brought the issue in to the public arena as never before and was interpreted as ending slavery in England, which is what Mansfield himself had believed it might do. The case remains Lord Mansfield's legacy as a key milestone in the abolition of slavery and one of the finest examples in English law of the maxim he himself quoted as a warning to the parties in the case before he began his months of deliberation - "let justice be done whatever the consequences".
The way those opposite ambitions were subsequently fulfilled in the Constitution of the United States without once using the words "slave" or "slavery" is well known, as is the later period of tension which saw the pro-slavery states making increasingly cynical efforts to maintain a legal basis for slavery. The decision of the King's Bench in Somersett's Case may be contrasted with the decision of the U.S. Supreme Court, some 85 years later, in Dred Scott v. Sandford 60 U.S. (19 How.) 393 (1857), where the U.S. Supreme court held that a black "whose ancestors were ... sold as slaves" was not entitled to the rights of a federal citizen and therefore had no standing in court, and that blacks were "beings of an inferior order" not included in the phrase "all men" in the U.S. Declaration of Independence nor afforded any rights by the United States Constitution. The case is also sometimes compared with North Carolina v. Mann, 13 N.C. 167 (N.C. 1830), in which the Supreme Court of North Carolina ruled that slaveowners could not be convicted for killing their slaves.
However, the letter is somewhat at variance with other sources reporting on the words of the Mansfield Decision (including the citation in the previous section of this article). Such inconsistencies are perhaps to be expected given the enthusiasm which abolitionists propagated the decision, and the spin which they sought to put on it in relation to their campaign.