Deja vu is defined as "the feeling that one has lived through the present situation before". To experience deja vu is to get the impression that what is a new, current, and ongoing experience in the present as being old, already experienced, and in the past. In essence, deja vu is to confuse the present (new) with the past (old) i.e. what one knows is a new experience one feels that one has already experienced it.

I want to discuss a certain theory of mine regarding deja vu and the first order of business is introduce the idea of block time which basically states that the past, present, and the future all exist together. That out of the way, consider if the theory of block time is true, my past self, my present self, and future self would all exist together.

Now, suppose my consciousnesses - past, present, and future - are connected to each other and information passes between them. If I, for instance, visit the Mahabodhi temple (Buddhism) in India for the first time and experience deja vu, the theory of block time provides a good explanation for it. The Mahabodhi temple is in the past of my future self i.e. my future self has already seen this Buddhist landmark but my present self is viewing it for the first time. If information (memory essentially) passes between my future self and my present self then the Mahabodhi temple will feel like a new experience (for my present self) but the Mahabodhi temple will feel like an old experience (for my future self). In other words, if one's future self and present self interact and exchange information, "the feeling that one has lived through the present situation before" will be experienced and that's deja vu.

Furthermore, deja vu has been linked to temporal lobe epilepsy and the temporal lobe, as the name suggests, is the part of our brain that's involved in time perception, making my theory just that tiny bit more plausible.

A penny for your thoughts.