If a person has any pre-knowledge of something that is yet to happen, like in a dream, it cannot be Deja Vu, and is likely to be some type of Precognition. (Even if the Precognition is only a few seconds before the event.) A person cannot know that a Deja Vu experience is coming, and also, it is also always sensed as INSTANTANEOUS, as being a sudden realization that an experience "was somehow familiar". This last part effectively eliminates Deja Vu from occurring in any place that the person has already been! The problem is that your mind has countless thousands of memories in it of all the days of all the previous years that you have lived, and, simply to maintain your sanity, your brain "forgets" most of them, even though they are still recorded somewhere in your brain. These "forgotten memories" are a well-known phenomena in all of us. So, if you are in a familiar environment, and you get a feeling that you "had experienced this before" the reality MIGHT be that you actually had! When your brain sees a PATTERN in an experience, it seems to have the ability to "suddenly remember" forgotten memories which happened to have a very similar pattern in them. This can therefore SEEM like it is a deja vu experience, of that spooky familiarity, but where the explanation is very mundane, simply a forgotten thought from three years earlier. Because forgotten memories are so common in us, then IF you are in a familiar environment when you get a sensation that you think might be deja vu, it is actually far more likely that it was simply a forgotten memory instead. Therefore, the credibility of a deja vu as being valid generally hinges on it being in an environment or situation that you HAVE NEVER POSSIBLY EXPERIENCED EVER BEFORE. Thus the described requirement of a new place. Yes, a deja vu COULD also occur in a familiar situation, but there is so strong a possibility that it would be a forgotten memory that any credibility as an actual deja vu must generally be dismissed.