In 1953 a patient known for decades as H.M. had both hippocampi largely removed to treat severe epilepsy. The seizures improved. He also lost the ability to form new lasting memories — while keeping most of his childhood ones, and keeping the ability to learn new motor skills without any awareness of having practised them.
That single case split memory into pieces that had previously looked like one thing.
Two systems, one event
When something happens, the cortex registers the details — sights, sounds, words, the feel of the room. The hippocampus rapidly binds those scattered fragments into a single retrievable episode. Early on, recalling the event requires the hippocampus to reassemble it.
Over subsequent weeks and months, during sleep and quiet rest, the hippocampus replays those patterns to the cortex, and the cortex gradually strengthens direct connections between the fragments. The episode becomes retrievable without the index. This is systems consolidation, and it is why H.M.'s older memories survived: they had already been transferred.
Retrieval is not playback
Recalling a memory reopens it to modification — a process called reconsolidation. The version you retrieve is influenced by everything you have learned since, by the questions you are asked, and by the state you are in.
This is not a design flaw. A memory system that updates old records with new information is more useful than one that stores fixed copies. It is also the reason eyewitness testimony grows less reliable each time it is rehearsed, and why confidence in a memory tracks how often it has been recalled rather than how accurate it is.