There are theories about  déjà vu. The most intriguing focuses on a neurological mistake. Information is supplied to the brain through many avenues- the eyes, the ears, the skin, the air we breathe in. Each and every piece of information must be collated and translated into a singular picture of the world in front of us. 

This information is delivered to both hemispheres of the brain, but it is the left hemisphere that is responsible for putting a time stamp on that information, it is the temporal lobe of the left hemisphere that tells you if what is happening is happening right now, or if it’s a memory. 

Sometimes when that information is passed from the right to the left, there is slightly longer than usual delay. The left hemisphere has already dealt with it’s version of the present situation but now is being supplied more of the same information, like an echo, coming nano seconds too late for it to be registered as the present, instead it is given the time stamp of the past. Your brain tells you it has all happened before. 

This particular kind of biological déjà vu is more accurately called déjà vécu - “already experienced.” It causes a sensation that we are observing ourselves- we suddenly become disconnected from the present reality, something so startling that we are obliged to stop and tell everyone present. This might be our way of resetting the connection, proving to ourselves that we are not caught in a hallucination of the past. 

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