Great Works: La Bella Principessa (Late 15th century), unknown artist

Private collection

(LUMIERE TECHNOLOGY/PASCAL COTT/EPA)

This week we will be doing something a little different. Instead of the customary close-scrutineering of a single work, we will be asking ourselves slightly different questions around this work, fascinating issues relating to the subject of authenticity, and how much of a bearing the certain attribution of a drawing or a painting to a particular master (or mistress) may have upon whether or not we are able and willing to value it.

This late-15th-century profile drawing may or may not be by Leonardo. Much hinges upon the question of its attribution. If it were to be declared to be a Leonardo, it could be worth as much as £100m. Given that it is not (or not yet) a Leonardo, it exists in a kind of critical limbo, that huge shadowland occupied by so many unattributed paintings and drawings, and it is therefore worth a miserable fraction of that amount. Who would want to invest heavily in a lovely drawing in the hope that it might in time become a Leonardo?

Better, perhaps, that it not be associated with that name at all, than that it be regarded as a kind of might-have-been – or might-yet-be... If it were firmly declared to be a Leonardo by a posse of connoisseurs, something else would happen too. We would look at it differently. We would prepare ourselves for the fact that we were about to see a Leonardo – and what is more, a new Leonardo, which would be such an astonishing thing, given that there are so few Leonardos in the world – and all that such an act of looking would draw along in its wake. We would regard it with a degree of reverence that it does not yet receive (or deserve?). We would pore over it, compare it with other works by the master...

Yes, some Leonardo scholars genuinely believe this work to be genuine. Others do not. It first came to the public's attention in 1998, during a Christie's sale. At that point it was said by experts at the auction house to be a 19th-century work, by a German artist. The owners were furious. Various tests were carried out, on the materials used – the vellum on which it is painted, the pigments, for example. The vellum was datable to the correct period – but that proves nothing, of course. A contemporary forger of works by Leonardos could have painted it.

Now let us consider the possibility that it is indeed a forgery by a contemporary of Leonardo's. How valuable would that make it? Let us say there are two kinds of value, the value at the auction house, and the so called intrinsic value of the work. Let us call that second kind of value spiritual. Now if it were definitely proved to be a brilliant contemporary forgery, the equal of a Leonardo by another hand, how valuable would it then be? How materially valuable? How spiritually valuable? Let us take one step further in the direction of fantasy, a step that might have been taken by the likes of Jorges Luis Borges had he written up such a beguiling thesis as one of his inimitably teasing short stories.

Let us consider the possibility that the person who judged this work to be a 19th-century German copy was, finally, proven to be correct, and that we were later to discover an entire body of works by this same artist, all painted on 15th-century vellum as if they were Leonardos, and yet all of them brilliant variants, major or minor, upon a Leonardo painting or drawing. Would this group of 19th-century German forgeries, all from the same inimitable hand, be of much greater interest to collectors and connoisseurs than a forgery by a contemporary of the master? And how much spiritual weight would such a discovery have for us? In short, how much of a great work would it prove to be?

All these issues are given careful attention in Art and Authenticity, a book newly published by Lund Humphries in association with Sotheby's Institute of Art.