The emergence in the mind of previously learned information that is treated as a new, original idea is cryptomnesia. A fragment of a song or a line of poetry comes to you, for instance, that you think you have invented, until someone else informs you it was Seeger or Lennon. The act of remembering, without knowing that is what you are doing.
It was first used by the nineteenth-century psychologist Théodore
Flournoy, who studied mediums, psychics, and others. The ability to
generate vivid recollections of past lives under hypnotic regression
is, perhaps facilitated by cryptomnesia. From Greek kryptos,
“hidden,” + mnesia, “memory.”