As we walked away from the World Trade Center site (or as close to it as we could get), a woman on a cell phone turned the corner onto Washington Street and said "I'm there. All I can see is empty sky." That captured it perfectly for me. If you had never been to the World Trade Center before they were destroyed, this empty sky would be meaningless. But I kept looking up, half-expecting to see two enormous white towers shining in the cold December sun. In Lower Manhattan, absence of evidence is indeed evidence of absence.