Steven Johnson's new column

Steven Johnson is now writing a new column for Times Select readers of The New York Times.

Johnson's newest book, The Ghost Map, makes an easy-to-read and brilliant case for the role of experts and amateurs. Focused on the famous map of London that Dr. John Snow developed to help him prove that cholera was a water-born disease, Johnson shows how the doctor and others needed two kinds of knowledge - city-level and street level. They needed to bring a "birds-eye" view of disease transmission, as well as door-to-door knowledge of London's Soho neighborhood, in order to track the disease to its source. An interesting subplot in the book is the relationship between this new knowledge - the theory of water-born illness - and the fight to disprove the dominant theory of airborne (miasma) disease.

Questions for philanthropists: who are the birds-eye experts that you depend on and who knows what matters at street level? How can you bring both sets of knowledge to bear in addressing social challenges?