Little Question Three of Question Three
Whether the dead can in dreams admonish the living toward good or draw them away from evil, and whether credence should be given to their admonitions
It is asked whether the spirits or souls of the dead can in dreams admonish the living toward good or draw them away from evil, and whether the admonitions of the dead are to be observed and carried out.
To this it is answered affirmatively. For dreams are sometimes sent to the living by spirits of the dead and by souls existing in Purgatory, who ask that prayers be offered for them, as is evident in Book IV of the Dialogues.
Sometimes they are also permitted to introduce dreams that are admonitory, or even provocative toward good, or that call persons back from certain sins.
But concerning the souls of the damned, I do not think that they are permitted to send dreams to the living, although nothing prevents the living from dreaming even about the damned. This sometimes occurs through the craft of demons, to the end that the damned may be believed to be saved, or that the prayers and alms of the living may be expended upon them in vain.
In whatever manner the matter may stand, however, dreams about the dead are not to be observed by the living as though those living persons were always admonished through such dreams to aid the dead with alms and prayers.