Robert Calef was a cloth merchant in colonial Boston. He was the author of More Wonders of the Invisible World, a book composed throughout the mid-1690s denouncing the recent Salem witch trials of 1692–1693 and particularly examining the influential role played by Cotton Mather.

The Salem witch trials were a series of hearings and prosecutions of people accused of witchcraft in colonial Massachusetts between February 1692 and May 1693. More than 200 people were accused. Thirty people were found guilty, nineteen of whom were executed by hanging. One other man, Giles Corey, died under torture after refusing to enter a plea, and at least five people died in jail.