Mather wrote a letter to one of the magistrates in the Salem witch trials, John Richards of Boston, urging caution in the use of spectral evidence. He was also the author of the "Return of the Several Ministers," a report sent to the judges of the Salem court. This document advised caution in the use of spectral evidence because the devil could indeed assume the shape of an innocent person, and degrading the use of spectral evidence in the trials, their "noise, company, and openness", and the use of witch tests such as the recitation of the Lord's Prayer.
