A library which is arranged in alphabetical order is no longer sorted but left to the users.

What will happen if every user picks a random place where he puts "back" his book?

In the beginning you knew exactly where to find a certain book. Now this knowledge decreases gradually. If you have only 20 books there are already 
2 400 000 000 000 000 000
possible sequences only one of which is correct..

Experience shows that alphabetical order does not develop by itself.
The value of the library decreases if no-one intervenes.

[auf deutsch]