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?

Answer:
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..

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

[auf deutsch]