Indexes and a second-hand book shop
Recently I was browsing in a second-hand book shop in Heidelberg. It's one of the oldest university towns in Europe, and the shop had a fantastic collection of ancient, old, and recent books. Unfortunately, I only had time for a short visit with my tired family wanting to get home (to our hotel) sooner than later.
So I went straight to the index pages of books with titles of interest. And to find those books, I looked around the shelves to see how the shop organised it's stock... with shelf labels providing a first level index. I soon had three books in hand, and had to stop.
Finally, at the counter, the shop assistant racked his brains for something kind to say, and observed that he had studied the subject of one my books (ethnology) and knew someone doing research in that area, and then pulled an unfamiliar name out of his mental index.
Written indexes are a natural extension of the mental indexes we all keep in our own heads.
We navigate the world by classification and association, and indexes are basically ways of organising labels (classifying information) and creating useful associations.
In that Heidelberg bookshop, I felt like I was wandering about inside the mind of a city that has been populated by students and scholars for many generations. My stay was very short, but I could meet a lot of people in a short time.