Suppose you were given a complimentary stay in a top hotel. Could you really enjoy this, unless you first knew how it felt to have no option other than to sleep on the street? Why, of course! If the memory of the street was powerful enough, you might perceive exquisite luxury in much more meagre accommodation. There is nothing like knowing how bad things can be to heighten recognition of how much better they have become. But for you to be blessed with the best this week, you don't have to start with the worst.