FRANKFURT  Here at the Städel Museum “The Master of Flémalle and Rogier van der Weyden” is an old-fashioned whodunit. Almost exhaustingly erudite, it mixes up very great Netherlandish paintings of the 15th century with a few not so great ones to unravel perennial questions from galaxy academe about which artist painted what.

Why should we care? For the same reason film buffs debate if Howard Hawks was the real director behind “The Thing From Another World,” the sci-fi classic from 1951 he produced, rather than Christian Nyby, the credited director, or whether the 1943 thriller “Journey Into Fear,” for which Norman Foster is listed as director, was taken over by Orson Welles, who played a Turkish police detective in it and whose other movies it partly resembles.

We should care because, commerce and the usual scholarly nitpicking aside, the debate is itself an excuse for looking closer, and because piecing together any great artist’s legacy is a bit like composing a novel, every chapter part of the artist’s grand narrative, without all of which the story is incomplete. And, well, also because good mysteries beg to be solved.

With 500-year-old Netherlandish paintings, their solution boils down to connoisseurship, an unfashionable term in universities for years, an art of human frailty that science and history now often miraculously supplement but still can’t replace. Like all forms of art, connoisseurship contrives its own logic to provide answers to questions about who did what, but it ultimately accepts doubt and ambiguity in the bargain, qualities which likewise separate theater from agitprop, “The Rules of the Game” from “Deuce Bigalow: European Gigolo,” literature from bank statements or grocery lists. Not coincidentally, writers like Tolstoy, Baudelaire and Zola used to love to write about art. The Goncourt brothers embraced Watteau, Proust took up Chardin. This exhibition virtually calls for a Borges or a Pirandello  or a Welles. Jochen Sander is its curator. On a recent afternoon he was standing before the “Crucified Thief,” attributed to the Master of Flémalle, a picture that is at once opulent and fairly revolting. Against a gold backdrop, two figures attend the badly mangled victim. In a corner of the picture a receding landscape abuts the backdrop, like a stage set running into the proscenium curtain, realism clashing with artifice.