This was a very short story. I assume it's early sci-fi but can't be sure.
The plot was two men sitting at a table having coffee. One of the men goes into the future with a notebook and camera. He returns and both items are empty. He is asked what he remembers and he says he remembers nothing. He then remembers that they "showed him everything and told him everything and then asked if he wanted to forget." The other person responds "Incredible."