I read a report that a person's brain can completely accommodate the knowledge of all books in a library, but why can't it be done in reality? The reason is that it is too slow to really remember that knowledge, and it takes a lot of time to deepen the impression. If you don't review regularly, you will always forget. Extreme examples: Alzheimer's disease, brain atrophy and other diseases are not that the original memories in the brain have really disappeared, but that the connection with these memories has been broken.