How much less valuable is a copy than an original work of art? Consider Salvator Mundi, the painting that went to auction at Christie’s in November 2017. In 2005, when it was thought to be one of multiple copies of a lost Leonardo da Vinci work, it sold for just $10,000. Then a group of art historians caused controversy by declaring that the Florentine master had himself painted it.

Among them was Martin Kemp, who commented that the work possesses an “uncanny presence” similar to other da Vincis—funny, considering that this quality is, by definition, difficult to pin down.

Even more complicated was the fact that extensive restoration had been undertaken to improve upon its cracked canvas and layers of overpainting, meaning that—whatever its origins—the work had undoubtedly b...