In his recent, and superb, essay on Philip Guston’s paintings — with rightly scathing comments about their cautious, halfhearted exhibition at Boston’s Museum of Fine Arts — my colleague Sebastian Smee observed how often 20th-century artists were drawn to “fragments, rubble and ruin.” More broadly speaking, many incomplete forms — the preliminary sketch, the rough outline, the abandoned draft, the catchall notebook — seem to offer entry into an artist’s true self, to guarantee (a perhaps factitious) sincerity and authenticity. Lacking finish and sophistication, such works expose what Yeats memorably called “the foul rag and bone shop of the heart.”