One of the great injustices of the Salem Witchcraft Trials was the admission of spectral evidence.

Spectral evidence was testimony that the accused witch's spirit (i.e., spectre) appeared to the witness in a dream or vision. The dream or vision was admitted as evidence. Thus, witnesses (who were often the accusers) would testify that "Goody Proctor bit, pinched, and almost choked me", and it would be taken as evidence that the accused were responsible for the biting, pinching and choking even though they were elsewhere at the time.

Thomas Brattle, a merchant of Salem, made note that "when the afflicted do mean and intend only the appearance and shape of such an one, say J. Proctor, yet they positively swear that J. Proctor did afflict them; and they were allowed to do so; as though there was no real difference between J. Proctor and the shape of J. Proctor."

Rev. Cotton Mather argued stenuously that it was appropriate to admit spectral evidence into legal proceedings. Robert Calef published More Wonders of the Invisible World to criticize Mather for this position. (Mather sued him for libel, and had the book burned in Harvard Yard). Cotton Mather remained unrepentant for his role in the trials and the admission of spectral evidence till his death.

Rev. Increase Mather, Cotton's father, however, became an opponent of spectral evidence - though not until after the Salem hangings had taken place, and not on the basis that it was false testimony by witnesses, but that it might be a deception by demons. He published Cases of Conscience Concerning Evil Spirits Personating Men, in which he argued that "It were better that ten suspected witches should escape, than that one innocent person should be condemned".