Actually why I'm writing today is because Andrea said something that reminded me that there is an issue I've had on my mind to write about for some time. When I talk about 'presence' (place illusion or plausibility) I always put a rider 'in spite of knowing that this is an illusion'. I.e., place illusion is 'the illusion or sensation of being in the virtual place in spite of the sure knowledge that you are not there' or plausibility is 'the illusion or sensation that what is happening is real, even though you know that there is nothing real happening'. Why 'even though you know that it is an illusion'? This was pointed out to me by someone as being redundant, or even wrong. But I had the feeling that it was a necessary part of the definition. This is why ….
It is because the feeling that is associated with the knowledge that the virtual experience is an illusion is part of the overall sensation associated with the illusion (e.g. of PI or Psi). Let's imagine that someone did not have knowledge that it was an illusion. Then for them there is no question of 'place illusion' since as far as they are concerned that are in that place, just as they would be in any place in everyday life. Hence there is no feeling associated with an illusion, since subjectively there is no illusion. Now it is quite a different issue to engineer a situation where participants do not know that it is an illusion. This is equivalent to subjectively 'reproducing reality' for that person. From a scientific and engineering point of view this is a more profound objective, and currently unachievable. Even in the famed 'holodeck' of Startrek people know that what they are experiencing is an illusion, unless it were possible to engineer in them the 'forgetting' of the fact that they entered from the real world into the holodeck, and the suppression of their knowledge about how the holodeck works. So in a sense the more modest goal is to get people acting in a VE in a realistic manner *even though they know that it isn´t real* which is different from the goal of getting them to falsely 'know' that it is real (and therefore the question of trying to get them to act realistically wouldn't arise, since by definition if they didn't know it wasn't real they would act realistically). At one level this seems like 'hair splitting' and only philosophical, but I think it actually reflects different engineering goals, as well as scientific ones. An analogy would be that to remove that part of the definition is equivalent to saying that we could make virtual reality operate like a dream. While the dream is happening you do not know that it isn't real - it is only afterwards when you wake up that you realise this. I think that at the moment this objective isn't realisable - except perhaps by the use of drugs, hypnotism, or neural implants.