In the case of the Bruegel, the signature was not the only argument for saying he did the picture. As with a few other works Bruegel painted, “The Wine of St. Martin’s Day” is done in tempera on fine linen, the pigment mixed not with egg or oil but glue. What results is a fragile matte surface from which paint gradually falls away. Even with the later varnish removed, a gauzy scrim seems to cloud the remaining image. Glue from a liner long ago added to the back of the canvas has also caused parts of the picture to pucker and bulge.

So the painting wasn’t easy to decipher, but, on close inspection, not withstanding the damage, it still looked exceptionally beautiful, almost more so for being fragile and ghostly. In the clear light of the conservation studio, you can admire the delicacy of faces and hands and feet, alive and varied, making a jigsaw of humane detail, Bruegel’s trademark: the cripple kneeling at St. Martin’s feet; the mother gulping wine with a baby still clasped to her breast; and the fallen drunk, limbs bent and splayed like a rag doll, face in the dirt.

Copies and an engraving based on the picture further obscured its probable link to Bruegel by attributing the image to his elder son, Pieter the Younger, whose studio turned out dim copies of the father’s art, or else to Jan, Bruegel’s other son. The former chief curator of Flemish painting at the Prado, having never seen “The Wine of St. Martin’s Day” except in reproduction, published an article in a Spanish journal in 1980 that also attributed it to Pieter the Younger.

But the other morning, Gabriele Finaldi, the Prado’s deputy director, recalled having noticed the painting two years ago. Its owner, a young heiress to the historic Medinaceli family, invited Mr. Finaldi to examine a different work that she wanted to sell. He told her he was intrigued by the Bruegel. A year later Sotheby’s, acting on the owner’s behalf, requested an export license to sell it abroad, and the Prado, unsure about the attribution, asked to inspect the picture first.

Privately, dealers are always boasting about spectacular finds: an unknown El Greco in a country home here, a long-lost Rubens in a private collection there. To ask the original question another way: Why do we want these works to turn out to be by Velázquez and Michelangelo? After all, the art is the same either way.