Although Minister Cotton Mather was not directly involved with the trials that took place in Salem, in 1693, he did write a letter to magistrate John Richards of Boston. In this letter, Mather express his concern with the magistrates considering to use evidence, what he called 'spectral evidence'.
He explained in the letter, that the devil could take the form of angels of light, innocent people, and even children, to persuade anyone. Therefore he was strongly suggesting the magistrates to be cautious when using this form of evidence during the trials.Minister Mather also added that using 'witch tests' as another form of evidence, was as dangerous and sensitive as the use of 'spectral evidence'.
Unfortunately for Mather, the magistrates took his later as an approval of the before mentioned forms of evidence, and the court decided to proceed with trial, accepting this forms of supernatural evidence as valid.
Mather was a central piece in the trials, because he was a scholar that had previously written and published work related to witchcraft, as his work called Memorable Providences, published in 1689, which recollected information about children being possessed by demons in Boston.
He was a fervent believer in witchcraft, and spent a large amount of his career trying to 'hunt' and expose the dark arts and the people that, in his opinion, were conducting such. That is the reason he was so involved with the trials, and certainly why he wrote that letter to the magistrate.