nation that is premised on an idea—not on an alleged shared bloodline or eons of history on common acreage—is prone to periodically question exactly who and what it is. The matter that binds Americans, as much as any doctrine or document, is the pursuit of a definition of who Americans are. There are facile adjectives applied to us—optimistic, volatile, swaggering—but they more often seem to apply to pretensions that we wear before the world. Who we are in our unguarded moments, and even what portion of people are included in the word we, is another matter entirely. This is part of the reason that Robert Frank’s photographic essay “The Americans,” published in France in 1958 and released in the U.S. a year later, is both an indelible reflection of American culture and one of the works that helped define it. To produce it, Frank, who died this week, at the age of ninety-four, spent two years scouring the country in a used car, courtesy of a Guggenheim grant, a contrail of dust his most constant companion.