First, the Church forbids us to conjure up the dead (Catechism
2116-2117). Peter Kreeft in his book Everything You Ever Wanted To Know About Heaven
(pgs 34-35) says the reason for “this stricture is probably protection against the danger of deception by evil spirits. We are out of our depth, our knowledge, and our control once we open the doors to the supernatural. The only openings that are safe for us are the ones God has approved: revelation, prayer, His own miracles, sacraments, and primarily Christ Himself…The danger is not physical but spiritual, and spiritual danger always centers on deception.”
“Nevertheless, without our action or invitation, the dead often do appear to the living. There is enormous evidence of “ghosts” in all cultures. What are we to make of them?” He goes on to say “We can distinguish three kinds of ghosts, I believe. First, the most familiar kind: the sad ones, the wispy ones. They seem to be working out some unfinished earthly business, or suffering some purgatorial purification until released from their earthly business. These ghosts would seem to be the ones who just barely made it to Purgatory, who feel little or no joy yet and who need to learn many painful lessons about their past lives on earth.
Second, there are malicious and deceptive spirits—and since they are deceptive, they hardly ever appear malicious. These are probably the ones who respond to conjurings at seances. They probably come from Hell. Even the chance of that happening should be sufficient to terrify away all temptations to necromancy."
"Third, there are bright, happy spirits of dead friends and family, especially spouses, who appear unbidden, at God's will, not ours, with messages of hope and love. They seem to come from Heaven. Unlike the purgatorial ghosts who come back primarily for their own sakes, these bright spirits come back for the sake of us the living, to tell us all is well. They are aped by evil spirits who say the same, who speak 'peace, peace, when there is no peace'. But the deception works only one way: the fake can deceive by appearing genuine, but the genuine never deceives by appearing fake. Heavenly spirits always convince us that they are genuinely good. Even the bright spirits appear ghostlike to us because a ghost of any type is one whose substance does not belong in or come from this world. In Heaven these spirits are not ghosts but real, solid and substantial because they are at home there: One can't be a ghost in one's own country."
“That there are all three kinds of ghosts is enormously likely. Even taking into account our penchant to deceive and be deceived, our credulity and fakery, there remain so many trustworthy accounts of all three types of ghosts - trustworthy by every ordinary empirical and psychological standard - that only a dogmatic prejudice against them could prevent us from believing they exist. As Chesterton says, 'We believe an old apple woman when she says she ate an apple; but when she says she saw a ghost, we say 'But she's only an old apple woman.' A most undemocratic and unscientific prejudice."
Everything You Ever Wanted to Know about Heaven
By Peter Kreeft
Ghosts and Poltergeists
By Herbert Thurston, SJ
Seven Kinds of Ghosts
Seven Kinds of Ghosts (article)
By Bro. John-Paul Ignatius