The island of Giudecca, a two-minute boat ride from the main part of the city, is so low-key and neighborhoody. No throngs of tourists. No guided crowds led by a flag-bearing leader. You can walk back alleys for hours and not see anyone except maybe a housewife on her way home with the makings of the evening meal, a boy kicking a soccer ball and talking to himself. One of the things I saw was this: a pair of old, much-repaired shutters, closed tight and emblazoned with a heart and the legend, “I think of you.” Who wrote this, I wonder, and about whom?