One of the few non-commercial disputes relating to slavery arose in R v Stapylton (1771, unreported) in which Lord Mansfield sat. Stapylton was charged after attempting to forcibly deport his purported slave, Thomas Lewis. Stapylton's defence rested on the basis that as Lewis was his slave, his actions were lawful.
Lord Mansfield had the opportunity to use a legal procedure at the time in criminal cases referred to as the Twelve Judges to determine points of law (which were not for the jury) in criminal matters. However, he shied away from doing so, and sought (unsuccessfully) to dissuade the parties from using the legality of slavery as the basis of the defence.
In the end Mansfield directed the jury that they should presume Lewis was a free man, unless Stapylton was able to prove otherwise. He further directed the jury that unless they found that Stapylton was the legal owner of Lewis "you will find the Defendant guilty". Interestingly, Lewis was permitted to testify. The jury convicted. However, in the course of his summing up, Lord Mansfield was careful to say "whether they have this kind of property or not in England has never been solemnly determined."
Read more about this topic: Slavery At Common Law