Prior to the French Revolution, ancient philosophers wrote about tyrannicide, as tyranny was seen as the greatest political threat to Greco-Roman civilization. Medieval philosophers were similarly occupied with the concept of tyranny, though the analysis of some theologians like Thomas Aquinas drew a distinction between usurpers, who could be killed by anyone, and legitimate rulers who abused their power—the latter, in Aquinas' view, could only be punished by a public authority. John of Salisbury was the first medieval Christian scholar to defend tyrannicide.