ALL THE most subtle and stunning qualities of Vermeer's paintings -- the mysticism, the sensuality, the passion, the wit and the deliberate intelligence -- are brought together in a brilliant argument for the crucial role of the artist in "The Art of Painting," the allegorical masterpiece that Vermeer himself kept adamantly private -- and which is the closest thing to a declaration of purpose he left behind.

The uncharacteristically large (nearly four feet tall) canvas was to have been a centerpiece of the National Gallery's Vermeer exhibit four years ago, but was too fragile to travel from the Kunsthistoriches Museum in Vienna; now restored and surrounded by four of the Gallery's own Vermeers, it has a polemical authority that adds new drama to his often enigmatic portraits.

Superficially, it shows an artist -- not a self-portrait, but an Everyman of the canvas with his back turned to the "audience" -- just beginning a painting of a girl who represents Clio, the Muse of history. Incongruously crowned with a laurel wreath, clutching her props and simpering, she stands in the light of an unseen window before an antique map of the Netherlandish empire. She seems a weak goddess. The painter's figure, big and brash in a slashed black velvet doublet, is an energy source that clearly dominates the subservient model.

But the more you gaze at the painting, the more the metaphors pile up. On one level, "Painting" is a master class on Vermeer's own technique, with its claustrophobicly precise perspective, its shifts of hard and soft focus and its soft folds of exquisitely diffused light. (It also passes along more mundane technique, showing the canvas's underpainting, the figure's chalked-in outline, etc.) It employs a sort of painterly rhetoric, using the shifting degrees of focus as stage directions for the viewer's attention: This is essential, this is plastic, this is faded (made irrelevant) by time.

Its central pun proclaims the "divinity" inherent in the creative process, in the transformation of an unlettered girl in not-so-classical drapery into an avatar of Dutch national pride. Beyond that, it bows to the artist's pride in his own cathartic part in this cultural preservation; and even beyond that, pays homage through the act of artistic creation to the original Creator.

And all the while, Vermeer is luxuriating in the resonance of his brush strokes, emphasizing the tension in the anonymous painter's shoulders (he seems just about to take a deep breath and plunge into the real work), the obvious weight of the endearingly kiltered chandelier, even the self-consciously portentous bit of ruined statuary. "The Art of Painting" is both proclamation and pure physical celebration. No wonder he never let it go.

THE ART OF PAINTING -- Through Feb. 6 at the National Gallery of Art, West Building,Sixth and Constitution NW (Metro: Smithsonian, Archives/Navy Memorial). 202/737-4215 (TDD: 202/842-6176). Web site: Open Friday-Saturday and Monday-Thursday 10 to 5, Sunday 11 to 6. Free.