What happens when we dig into the past?

Agatha Christie tells a story about the danger of not leaving the past alone. Miss Marple is explaining that a young man was looking through his mother's attic shortly after she had died and discovered an old award, a framed parchment. He wanted to know more about how she got it so he did some research and found . . .

"Then she hadn't really won it?"

"No," said Marple. "She wasn't really his mother."

The joys and the pain that we discover when we learn about the past are often mixed blessings.

We think we know what happened. We were there, after all. The summers in the country. The first day of school. Falling in love -- how could we forget? It is all so clear in our mind until we stop to examine it and then . . .

Memory is a mirror. We see ourselves and we recognize what we see. It is familiar and seems close. But when we reach out to touch it we find that the mirror is not glass. It is soft like water. And when we make contact with it we also make ripples and the image is broken up and confused.

The waves last a long time. The mirror creates new images. The picture is rearranged and new connections made.

The quotation above is just a paraphrase from memory. I don't know the name of the book or story in which it appears. If some reader could supply this detail, I would be grateful.