James Somersett or Somerset was a slave who was brought by his owner from Virginia to England. He then escaped, and was recaptured and bound in irons for shipment to Jamaica. In 1772 the Court of King's Bench issued a writ of habeas corpus requiring the ship's captain to bring Somersett to court. The chief justice, Lord Mansfield, issued a judgment which concluded:

"...The state of slavery is of such a nature, that it is incapable of being introduced on any reasons, moral or political; but only positive law, which preserves its force long after the reasons, occasion, and time itself from whence it was created, is erased from memory: it's so odious, that nothing can be suffered to support it, but positive law. Whatever inconveniences, therefore, may follow from a decision, I cannot say this case is allowed or approved by the law of England; and therefore the black must be discharged."

Lord Mansfield is often misquoted as declaring that "The air of England is too pure for a slave to breathe", however, 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

"...one Cartwright brought a slave from Russia and would scourge him; for which he was questioned; and it was resolved, that England was too pure an air for a slave to breathe in."

Somersett's case (Lofft 1; 20 Howell's State Trials 79) decided conclusively that slavery could not exist in England. Nevertheless, it was not until 1807 that Parliament abolished the slave trade; and slavery continued to exist in various parts of the British Empire until it was finally abolished by Act of Parliament in 1833.