SWIM used to have some pretty intense encounters with ghosts when he was taking anahuasca brews. The encounters tended to occur between sessions, rather than on them. The first serious encounter occurred within twenty-four hours after a session.

One night, SWIM was having a rather pleasant dream, but at some point in the dream an intruder began to pummel SWIM until he woke up. SWIM was sleeping in perfect pitch dark at that time, and when he woke up, he observed a very faintly luminescent entity. The entity was like a disembodied head, except the head was its entire body, so it wasn't as if it were decapitated. The interesting thing about the entity is that it seemed to exist independently of SWIM's perspective: SWIM could look away and then look back at the same location and it would still be there. It even began to fly around the room as SWIM attempted to catch it until it disappeared.

Subsequent "ghosts" have primarily been of humanoid form but have been of a more shadowy character since SWIM stopped taking the brew. SWIM's finally getting into proper pharmahuasca preparations and expects a reemergence of these types of encounters.

SWIM doesn't believe in hauntings but considers the possibility of some sort of collective unconscious being able to manifest through hallucinations of many kinds.

Finally, a topic on ghosts! Being physically attacked is uncommon. Ones that attack sleepers have precedent... although more often they're associated with sleep paralysis- the user wakes up, and the entity is holding them down so that they can't move. I think I heard that in folklore they kill you if you don't wake up. It was believed in Britain that they are witches so they were depicted as hags. However, I personally have woken up from dreams and imagined things on the odd occasion, like thinking there's someone in the room but then it turns out to be the dressing gown hanging on the wall... your brain fills in blanks when surroundings appear ambiguous, particularly in the dark. I can imagine the ayawaska use heightens this effect. I'm sure that since my foaf's been using cactus, ayawaska and spice, although infrequently, his vision has altered slightly, as if he notices the visual anomalies that his mind used to ignore. He likes it, it's pretty.

I saw a ghost when I was about 7 years old. It was of a monk dressed in black robes, with only darkness where his face should've been. He was standing over a metre above the ground, I would say hovering but that would imply movement, and seemed to be watching me. This was in the middle of a sunny summer day, and he was as concrete as I was. I was paralysed with fear for what seemed like ages, then finally was able to snap out of it and ran away to tell my friend and mother. I was sound in mind as a child and never had any mental problems. When I was older, I found out that this type of ghost is called a 'guardian' and is associated with graveyards. I had seen the ghost in the corner of a garden with a graveyard the other side of the garden wall, 4m from where the ghost was.

I was already developing atheist tendencies at that age, but this encounter made me think that there might be more to life than science thinks. I'm totally open to scientific suggestions, in fact I actively search for them. This experience had a profound effect on me.

I have read that old houses emit inaudible low frequencies, which makes people feel the uncomfortable feeling that they attribute as being a 'presence'. I watched a documentary where they monitored people in 'haunted' Scottish catacombs- the people felt presences and got very scared. Then they made a computer simulation of the catacombs... and found that people watching those got scared and felt presences too! Suggesting that it was a spatial effect creating these feelings. However, I've come across no provable theories for when people actually and unquestionably see ghosts, rather than imagine them or think they saw something out of the corner of their eye. I have heard theories about electromagnetic fields causing hauntings. I'd love to hear of any other possible scientific explanations. Conventional science stays well away of investigating ghosts, I think they're scared of being associated with anything like that, it's almost a taboo in these times. Also, there's no money in it I guess.

By chance I just saw that the 'Reasoning' section of the Wikipedia entry for 'Machine Elves' talks about both sleep paralysis and your mind filling in blanks:

http://en.wikipedia.org/wiki/Machine_elf