I n May, partners of the Gates Foundation gathered in Manhattan to announce that the organization would spend $200 billion over the next 20 years and then close its doors in 2045. Supporters, including Michael Bloomberg , were on hand to mark the occasion. The scene, staged at Carnegie Hall, a venue built by one of America’s great 19th century philanthropists, paid tribute to a long tradition of American giving, while pointing to new ways of thinking that are shaping the 21st century. For those