Cotton Mather was an exceptionally prolific author, who wrote on a diverse range of subjects. Many of his writings were historical, and others were scientific. He was a leading authority on vaccinations, particularly for smallpox, a disease of which some of his children died. His writings on vaccination were controversial and led to his being attacked in print and threatened with violence.
It is for his writings on witchcraft, however, that Mather is now best-known, as well as the peripheral role he played in the Salem witch trials. By 1692, Mather had already investigated several alleged cases of witchcraft and published a book called Memorable Providences, in which he refused to accept scientific explanations for such occurrences as hysterical fits, insisting that they were the work of the devil. He also played a leading role in the trial of Ann Glover, the last person to be hanged for witchcraft in Boston, in 1688.
Opinions are divided as to how significant a role Mather played in inciting the hysteria that attended the Salem witch trials. In his book More Wonders of the Invisible World, written shortly after the trials, Robert Calef suggests that Mather's writings were highly influential. Even if Calef exaggerates, however, there is no doubt that Mather's writings inflamed public opinion rather than calming it.