IN THE PRESENT-DAY REALM OF ART, confusion proliferates between public and private, between profit and nonprofit. Commercial galleries mount loan shows that would distinguish any museum, while museums mortgage themselves in the service of privately amassed collections, and collectors rebrand their possessions as museum holdings. Entangled interests make for endless ethical quandaries. But the National Gallery of Art in Washington, DC, has mounted an indispensable exhibition that celebrates a collector and commercial gallery, yet revives disinterested probity as an example for our current moment.