There are so many answers to this that will lead to other questions. For instance, reincarnation. Whether you believe it or not, if you have watched or studied evidence of it, that could lead to the ultimate discovery of this deja vu. You could have had a dream about it, if you know it’s something common to expect. There is also astral projection. Which is when the “soul leaves the body” in a sleeping state. You don’t have to be asleep, but that’s normally when. This is also when split personalities will occur. Apparently, a demon can take place of your soul enabling this split personality to occur. These could be possibilites to deja vu. However, I believe that lucid dreaming could be part of the cause of deja vu. Which is when the conscious mind is working in your sleep. It’s all really a matter of faith. Whether you had a past life or not. If any do believe in this, it’s the belief that when reincarnation occurs no memories are passed on to the next life, usually. If they are passed on, they are only remembered at a young age. So to say, 2–4 years old. They soon out grow this though. Which is why I believe we do not remember much of our infant lives. Anyway, what is actually transferred are signals, such as deja vu. Scientifally speaking, deja vu usually only occurs when you are thinking about the future, or anticipating it. Makes some sense, huh? There’s also this thing called Jamais Vu. Basically, it’s just the opposite of deja vu. Where in deja vu something seems completely familiar. Where with jamais vu, it seems totally unfamiliar. For example; you could be having a conversation with someone you’ve known for a very long time, and you feel like the person is totally unfamiliar. Jamais vu isn’t as common as deja vu though. How well you respond to either of these is whether or not you enjoy it, or if it bothers you. Some people become terrified, others are interested or amazed. These are pretty much my views on it. Anyway, hope this helped.