The court settlement in the Duveen case did little to alter the market’s opinion of “La Belle Ferronnière,” which remained unsold until 2010, when Sotheby’s attributed the painting to a follower of Leonardo’s and auctioned it for $1.5 million. (The New York Evening Post understood the court’s limitations back in 1929, when it asked in an editorial on the Duveen case: “How can anyone outside of a comic opera expect the authenticity of an old painting to be settled by a lawsuit?”)

Mr. Spencer, who edited the book “The Expert Versus the Object: Judging Fakes and False Attributions in the Visual Arts,” explained the disconnect between the culture of commerce and the courts. “In civil litigation the standard of proof is ‘more likely than not.’ Now picture yourself walking into a gallery and seeing a Picasso. You ask, ‘Did Picasso paint that?,’ and the dealer says, ‘Yes, more likely than not.’ You wouldn’t buy that.”

Just as a woman can’t be a little bit pregnant, a work of art can’t be a little bit real.

The classic example is a 1993 ruling by a federal judge that “Rio Nero,” a mobile ostensibly by Alexander Calder, was the real thing. Despite the decision the owners of this “genuine” Calder could not sell it because the recognized expert, Klaus Perls, had declared it a copy. Nineteen years later it remains unsold.

The judge recognized the problem at the time, noting that Mr. Perls’s pronouncement would make “Rio Nero” unsellable, but concluded: “This is not the market, however, but a court of law, in which the trier of fact must make a decision based upon a preponderance of the evidence,” or what is known as the 51 percent standard.

A 2009 opinion also involving a Calder stated the divide between the court and the market more bluntly. At issue were a couple of stage sets that Calder had designed but did not live to see completed. When the owner, Joel Thome, tried to get the Calder Foundation to authenticate the works so he could sell them, it refused. Mr. Thome sued and lost. The Appellate Division of the New York Supreme Court explained its rejection of Mr. Thome’s appeal by referring to “Rio Nero.” The fate of that artwork, Justice David B. Saxe wrote in his opinion, illustrates “the inability of our legal system to provide a definitive determination of authenticity such as is sought by plaintiff.” Having the court declare the sets to be authentic is meaningless, he told Mr. Thome, “because his inability to sell the sets is a function of the marketplace.”