If we were asked exactly what we were doing a year ago, we should probably have to say that we could not remember. But if we had kept a book and had written on it an aceount(记录) of what we did each day, we should be able to give an answer to the question. It is the same in history. Many things have been forgotten because we do not have any written account of them. Sometimes people did keep a record of the most important happenings in their country, but often it was destroyed by fire or in a war. Sometimes there was never written record at all because the people of that time and place did not know how to write. For example, we know a good deal about the people who lived in China 4000 years ago, because they could write and leave written records for those who lived after them. But we know almost nothing about the people who lived even 200 years ago in central Africa, because they had not learned to write. Sometimes, of course, even if the people can not write, they may know something of the past. For most people can tell proudly what their fathers did in the past. This we may call 'remembered history'. Some of it has now been written down. It is not so exact or so valuable to us as written history is, because words are much more easily changed when used again and again in speech than when copied in writing. But where there are no written re- cords, such spoken stories are often very helpful. Which of the following ideas is not mentioned in the passage?