Plato's Phaedo on Ghosts: 80d-82c

Ghosts are souls that are somehow stuck in their involvement in worldly things, and so are not in the best condition. From both the Dixboro Ghost and the ghost in Hamlet, we conclude that these are stuck regarding revenge, or the injustice of their murders and revenge toward their murderers. In the Dixboro story, one mark of authenticity- aside from the question of what ghosts are- is that Martha undergoes a Purgatory, as well as revealing one and preventing another a murder. At 81c-e of the Phaedo, Socrates tells Cebes,:

...But I take it she'll be set free pervaded by the body-like, which the company and intercourse with the body have made grow together with her because the soul was always with the body, and gave it much care?"

Of course"

And, my friend, we should imagine that the body-like is oppressive and heavy and earthly and visible; and the soul in the sort of condition we described is made heavy and dragged back into the visible region through terror of the unseen and of Hades and, as they say, circulates among the memorials and tombs around which certain shadowy apparitions of souls have been seen, ghostly images produced by the sort of souls that weren't released in purity but participate in the visible- which is why they too are visible

That's likely Socrates."

Of course it's likely Cebes. And it's not at all likely that these are the souls of the good- they're the souls of the inferior souls compelled to wander around such places paying the penalty for their former way of life, which was bad. And they wander about until, through the desire for body-like that stalks them, their again entangled in whatever sort of characters they happen to have made their care in life...

Socrates compares this sticking to what is said of reincarnation images, that one who acts as a donkey may become one. Those who practice vulgar virtue though without nous and philosophy get to enter nice animals like bees. Again, for Socrates, much of what WE call Christian and a matter of "faith," he calls philosophy, as "only the true philosophers recover their wings."