John Locke has a new article out, although he died in 1704. When England’s King Charles II discovered a failed 1683 assassination plot against himself and his brother, he beheaded the plotters and started arresting opponents. Among the targets was Locke, who knew some of the conspirators but likely wasn’t involved. Fearing for his life, Locke fled to the Netherlands. Before leaving, he deposited several manuscripts with Edward Clarke, a member of Parliament and trusted friend.

One of those manuscripts has only now been discovered—in...