Seth addresses the topic of "ghosts" in "Seth Speaks"

I thought this was interesting, and it's consistent with what I've read from reliable sources. In a nutshell, Seth ascribes many appearances of ghosts to projections of disturbed, deceased personalities who, unwittingly, send out thought forms to their former "haunts":

There are obviously as many kinds of ghosts and apparitions as there are people. They are as alert or as unalert to their situation as you are to your own. They are not fully focused in physical reality, however, either in personality or in form, and this is their main distinction. Some apparitions are thought-forms sent by survival personalities out of lingering deep anxiety. They portray the same compulsive-type behavior that can be seen in many instances in your ordinary experience.

The same mechanism that causes a disturbed woman, say, to perform repetitive action such as a constant washing and rewashing of hands, also causes a particular kind of apparition to return time and time again to one place. In such cases the behavior is often composed of repetitive action.

For various reasons, such a personality has not learned to assimilate its own experience. The characteristics of such apparitions follow those of a disturbed personality - with some exceptions, however. The whole consciousness is not present. The personality itself seems to be having a nightmare, or a series of recurring dreams, during which it returns to the physical environment. The personality itself is "safe and sound," but certain portions of it work out unresolved problems, and discharge energy in such a fashion.

They are in themselves quite harmless. Only your interpretation of their actions can cause difficulties.

One of Seth's core concepts is that our thought forms exist as tangible, though invisible, objects, much to our unawareness. These thought forms can be perceived by a few people. It would only be logical, then, that as we who are alive project our emotions and thoughts into non-physical realms, those who have passed on would project similar exteriorizations into the physical, which would then occasionally be witnessed by the living.

I wonder how EVP fits this model... While most EVP seem non-interactive, some EVP suggest a surprising time and situational awareness by the EVP'er. Still, I find Seth's explanation more reasonable than the truism accepted by most ghost hunters: that ghosts are the complete personalities of those who have failed to "pass on."

A bit later, Seth mentions that between lives there exists "a midplane of existence; a rest area, comparatively speaking" where personalities reside between incarnations, and it is from this area that "some disturbed personalities have those dreams of returning to the physical environment."

The concept is simple and elegant: As we, the living, often unwittingly project portions of ourselves in the dream or out-of-body state, so do the deceased.

Seth is somewhat vague about the nature of this "midplane of existence," but I tend to accept the findings of Dr. Michael Newton (and, to a lesser extent, those of his disciples).

Seth does indicate that this midplane would be familiar to those still living, because it is visited occasionally by the living in dreams, and communications with deceased friends and relatives are initiated here: "[I]t is from this area that most communication from relatives occurs. This is usually the level that is visited by the living in projections from the dream state." Elsewhere he states, "Some dead friends and relatives do visit you, projecting from their own level of reality into yours, but you cannot as a rule perceive their forms. They are not more ghostly, or 'dead,' however, than you are when you project into their reality - as you do, from the sleep state."

The notion that we visit the afterworld in dreams is common in metaphysical literature, and it's one that I've spent a lot of time debating myself. Quite regularly, I find myself wondering whether a specific dream is the product of such a visit, or merely a "dream." However, with several thousand recorded dreams under my belt, I can say that I've found more than several that have common themes and locales that consistently suggest that this has happened. And while my dream communications with deceased relatives are rare, to me, they definitely seem like real contacts to me. Certainly an interesting concept to ponder.

By the way, Seth hints that such visitations by non-physical personalities are quite common, but we ignore the signs:

Your rooms are full now of thought forms that you do not perceive; and again, you are as much a ghostly phenomenon now as you will be after death. You are simply not aware of the fact. You ignore certain temperature variations and stirrings of air as imagination, that are instead indicative of such thought forms. You thrust into the background telepathic communications that often accompany such forms, and you turn aside from all clues that other realities exist quite validly with your own, and that in the midst of one existence you are surrounded by intangible but valid evidence.

I am probably as guilty of this neglect as anyone else... having experienced so many oddities in my life--and because I know the dangers of succumbing to fears and superstitions--I err on the side of the material and rational. But in Seth's view, such paranormal happenings are as natural as we are, and they become unreal only when we willfully deny their existence.

Seth concludes: "The very words 'life' and 'death' serve to limit your understanding, to set up barriers where none intrinsically exist." I did not fully appreciate this sentence when I first read it, almost thirty years ago, but I am beginning to now. Advances in physics suggest that time is not the absolute that we perceive it to be; and the time barrier that seems to exist between us, the living, and those who have passed on, becomes less absolute with each passing moment.