Several years ago I began a review of Paul Johnson’s magisterial tome “Art: A New History” by telling of a walk I took with the author in Kensington Gardens. Johnson stopped to do some sketching and at one point burst into a denunciation of Picasso. I made the mistake of expressing admiration for “Boy Leading a Horse.” That precipitated The Great Johnson into a lecture on how sloppy, even cartoonish, was the boy’s left foot. “Just take another look at it,” Johnson commanded. “You’ll see.”

So I did this week — in Maine, where the Portland Museum of Art has put up an exhibition of William Paley’s collection of modernist masters. It’s rare that I’m within a hundred miles of Portland and don’t make a point of swinging by the museum, whose curators always seem to assemble art of quality and mount it well. Their Winslow Homer exhibition several years back was a case in point.

It’s hard to imagine any museum mounting a single small exhibit of canvases any choicer than those from Paley’s collection. These start with the first painting Paley bought, a small self-portrait of Cezanne. Several other beautiful Cezannes are in the exhibition, as well many other of the greatest names, such as, to mention but some, Andre Derain, Paul Gauguin, Alberto Giacometti, Juan Gris, Edward Hopper, Edouard Manet, Henri Matisse, Jackson Pollock, Odilon Redon, Pierre-Auguste Renoir, and Georges Rouault, Henry Rousseau, Ben Shahn, Henri de Toulouse-Lautrec, and Edouard Vuillard.

There is a particularly fabulous painting, in charcoal and pastel on transparent paper, by Degas. It is a large painting of two ballet dancers, beautifully framed and under glass so that one can lean reasonably close without fear of damaging it, and glimpse into the way his great paintings were formed. It is, at 43 inches by 32 inches, way beyond, say, the sketchbook stage of things. But it’s basically a charcoal sketch filled out with some dusky-red, put in with pastel on a paper with a warm yellowish tone.

It was tempting to bolt right then over to Deering Avenue to see if the Artist and Craftsman Supply was still open and might have the kind of paper Degas deployed (they just might, too, at that well-stocked store). But upon turning around I found myself face to face with what must be one of the greatest paintings by any artist of any time. This is Picasso’s aforementioned boy with the aforementioned horse. Let me just say, Degas, great as he is, can wait.

I had never actually seen the canvas in person. How much greater the impact of the real thing, which is some seven feet by four feet. For those who favor value painters, like Rembrandt, who used the interplay of darkness, shadows, and light to create his masterpieces, “Boy Leading a Horse” may take some adjustment, not that it’s without some marvelous use of value. But it is a painting of line and color and composition that evokes the classic statues of Greece.

Picasso captures — in the way the boy extends his right arm under the head of the horse, as if holding a rein — the gesture of command. He has set the gray horse off against a rose-colored landscape and scumbled a gray sky, all in tones of incredible beauty. As for the boy’s left foot, well, I would refer my friend and better, Paul Johnson, a giant who is the son of a drawing master who learned this art at his father’s knee, to Rembrandt.

The greatest of Rembrandt’s self-portraits, at least in my view, is an almost full figure from his latter years, and it’s all in the face, which is rich in color and value and texture and nigh perfect drawing. The rest of himself Rembrandt has crudely drawn and painted, including his own hands, which are resting on his hips (his arms akimbo) and appear almost to be in padded gloves. They could have been better drawn, but Rembrandt didn’t want to distract.

Nor, one can speculate, did Picasso want to send people looking at the curl of a toe when the painting is about the power and beauty of the boy and the horse. It is hard to imagine that one will have many comparable opportunities to see and appreciate this painting without crowds or distraction. It is there, alone on its own wall, at the Portland Museum of Art until September 8. It and the rest of the Paley paintings are worth a long drive. It would be worth a trip to Maine for this alone.

The William S. Paley Collection: A Taste for Modernism, through September 8 at the Portland Museum of Art, 7 Congress Street, Portland, Maine, (207) 775-6148, portlandmuseum.org.