Memory in reincarnation

The two blocks just below are for scientific explanation of the relation between memory and personal identity. You can skip it.

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"Personal identity is the concept you develop about yourself that evolves over the course of your life. This may include aspects of your life that you have no control over, such as where you grew up or the color of your skin, as well as choices you make in life, such as how you spend your time and what you believe."

[From: https://study.com/academy/lesson/what-is-personal-identity-definition-philosophy-development.html#:~:text=Personal%20identity%20is%20the%20concept,time%20and%20what%20you%20believe.]

"John Locke speaks of personal identity and survival of consciousness after death. A criterion of personal identity through time is given. Such a criterion specifies, insofar as that is possible, the necessary and sufficient conditions for the survival of persons. John Locke holds that personal identity is a matter of psychological continuity. He considered personal identity (or the self) to be founded on consciousness (viz. memory), and not on the substance of either the soul or the body."

[From: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3115296/#:~:text=A%20criterion%20of%20personal%20identity%20through%20time%20is%20given.&text=John%20Locke%20holds%20that%20personal,the%20soul%20or%20the%20body.]

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According to John Locke, personal identity is a psychological continuity and is based on memory. This is a very controversial. Thomas Reid will oppose this theory. For him, I quote: "Though memory is not the metaphysical ground of personal identity, it provides first-personal evidence of it.".

In any circumstances, memory consistency is a factor of personal identity.

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I demonstrated why memory is essential for maintaining one personal identity. Now why before reincarnation someone would willingly forsake his memories? Plot device.

I know that it's hard to create a backstory and maintain it in a new environment.

However, if someone willingly forsake his own memories (in its entirety), he is committing suicide. So far, it's not a problem just why would someone wanting to die would wish to reincarnate. The solution to this is obvious: make the memory erasing involuntary.

If the memory is erased (voluntary or not), why not just write the story about an orignal character?

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All personal memories are erased. This is also death. So it should be involuntary. Now this is done to imply that the reincarnated individual would use his external knowledge to impact the story (by copying techniques, by creating new technologies,...) It would also give him an original view of the world.

Now why not just giving these knowledge to an original character? It could be done nicely enough like with the Temporal Demon Spirit Book from Tales of Demons and Gods. This a book supposedly originating from another realm that contain knowledge and had some bullsh*t power to boost. Furthermore, how much knowledge can someone accumulate in a lifetime and how much of this knowledge would be relevant in a reincarnation hypothesis? Only viable solution is wishing for this knowledge. Then why not just use a plot device and give this knowledge to an OC and avoid claim of lame wish.

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Fragmented memories. I have no problem with this one. It serves two purpose: to justify eventual stupid action from the Main Character and to eventually regain all these memories.

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Keep the memories intact. This also creates a problem. With a whole set of memories from another world, the readers expect of you to create a whole backstory for the MC.

Now the best things to do would be to just write a backstory for the MC only that it really annoys me to read about it. Firstly, where to put it. If you put it at the start the story takes too much time to start. If you put it between the story (end of chapters or side chapters), 90% of the readers would just be annoyed by it and want more advance in the story. The ideal solution would be to mention the backstory each times there is a situation calling for it only that it makes it seems like the author is pulling it from his ass. Fortunately, all this sh*t only affects story where the reincarnated MC had an interesting (impactful) first life.

This takes us to the most common trope for reincarnation. MC is an average dude with no extraordinary feature and a weeb to have knowledge of the world where he is going or at least how things work in these settings [I would probably explain why it's a stupid (it's my subjective opinion) for a fanfic MC to have no knowledge of the world he's reincarnating in another chapter]. It's boring but it works.