What is Déjà Vu?
Have you ever felt that when you go to a new place, you feel that you have been there before but don't remember when you came there? or you are talking with your friends and you feel that you have talked about that topic before but you don't remember when? For some time you feel that the work you are doing or the present you are watching now had been already done or seen by you before, the place, things, people, and everything look the same. In this situation, you might get worried and think of how it is possible. If you had felt this type of situation then no need to worry because you are not the only one to feel this, two third of the whole population of the world had felt this. This feeling is known as Déjà Vu.
The term Déjà Vu was given by parapsychologist Emile Boirac. It is a French term that means 'already seen'. So, as the name, Déjà Vu suggests that it is a strange feeling that the current situation had happened before. Déjà Vu isn't a mental illness or a mental disorder, rather it is a mental state which many people feel in their life.
What Causes Déjà Vu?
Well, to know why Déjà Vu is caused, there are many theories given by scientists from which, few theories don't make any sense. All these theories are listed below:
Dream Based Theory:
Well, according to this theory the reason for Déjà Vu are dreams. Before sleeping, the things we think have a huge chance to come in our dreams and are mostly related to our real life. 
For example, before sleeping, you were thinking that the next day you will go to a picnic with your friends and these things come into your dream. As you might be knowing that after waking up we forget our 90 percent dream. And in reality, when you go to a picnic with friends, the dream starts coming in front of you and you feel that the situation has already happened. So, this was the dream-based theory.
Glitch Theory:
Well, this is a complex theory and hard to understand so, I will try to explain you in simple words. According to this theory, Déjà Vu is a very important part of our life and this theory denies dream-based theory. This theory says that time doesn't exist, it is an illusion that is used to ease the life of a person. Time is deceit and when Déjà Vu happens, we feel the past, present, and future at the same time. And by this, we get rid of this time illusion. As we feel the past, present, and future at the same time so we feel that the current situation had happened earlier.
Now, let us know about some of the psychological reasons behind Déjà Vu. According to psychology, Déjà Vu happens due to memory mismatch. To know the relation between our memory and Déjà Vu, we need to first know how our memory works. There are two parts of our memory, short-term memory, and long-term memory. In short-term memory, the memory is processed and then transferred to long-term memory. Short-term memory has a limited capacity, the information stored in it is for short time. On the other hand, long-term memory has an unlimited storage capacity, the memory of our whole life is present in it and could be recalled if we think properly. Sometimes, the information, without passing the short-term memory directly passes to the long-term memory, from which we feel that the current situation had happened before and this leads to Déjà Vu.
Another reason for Déjà Vu is the Conscious and Subconscious Mind. In normal cases, when the conscious mind is active then the subconscious mind should be inactive but a rare situation happens that when the conscious mind is active then the subconscious mind is also active with it. This is a rare situation and lasts for short period, at this time our mind becomes confused and is unable to find whether the present is reality or a dream, and for this reason, the mind feels that the current situation had happened earlier.
So, this was all about Déjà Vu. All the above-mentioned reasons behind Déjà Vu are theories so they may or may not be true. Well, there are more theories related to Déjà Vu but these above-mentioned are the most accepted. Some people relate Déjà Vu to parallel universe, reincarnation, and with ghosts but these are all superstitions. The real reason behind Déjà Vu isn't known yet.
~Thank you for Reading~
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Psychology