In Alan Bennett’s TV monologue, ‘The Hand of God’, a provincial antiques dealer is pleased to have sold an old picture frame containing a smudged drawing of a finger for a hundred pounds. To the dealer’s dismay, it later turns out that the customer wasn’t interested in the frame at all: he was an art expert from an auction house (‘some young blood from Christie’s’), and the drawing was an original sketch by Michelangelo for the Sistine Chapel ceiling. It is, of course, worth millions.

Lancelot Thwaytes must have experienced a similar dismay when Sir Denis Mahon, a world expert on Caravaggio, announced to the world that the painting he had bought from Thwaytes for £42,000 at a Sotheby’s auction was in fact an original by the master, worth in the region of £11 million. Sotheby’s had catalogued the painting (which Thwaytes had inherited) as being by a ‘follower’ of Caravaggio, with an estimate of £20,000 - £30,000. Unsurprisingly, perhaps, Thwaytes sued Sotheby’s for negligence.

Thwaytes v Sotheby’s [2015] EWHC 36 (Ch) came before Rose J (as she then was) in October/November 2014. It is a fascinating case on many levels: art history, the role of connoisseurship, the limits of technical analysis in questions of attribution, and the legal duties of auction-houses. Despite the attribution by Sir Denis (supported by a number of other expert witnesses), Sotheby’s firmly maintained that the painting was not by Caravaggio at all, but merely a copy of the famous ‘Cardsharps’, in the collection of the Kimbell Art Museum, Fort Worth. If the Mahon painting was a copy, Thwaytes would be unable to show any loss (unless he adopted the untenable position that he had been denied the opportunity to sell the painting to a speculative bidder for more that it was actually worth).

Much of the evidence at the trial was therefore directed to the question whether Sir Denis’s attribution was, in fact, correct. It is not, however, the function of the courts to determine as a matter of law whether a painting is an artist’s original or a copy. Nor should it be. Judges are not art experts: on issues of authenticity, all they can decide is whether, on the basis of the evidence put before the court, it is more likely than not that a painting is genuine. The legal question in the case was not whether the painting was in fact by Caravaggio, but whether the auction house had taken reasonable steps in coming to the conclusion that it was not.

Professor Richard E Spear acted as one of the expert witnesses called by Sotheby’s in its defence against Thwaytes, and he is firmly of the view that the Mahon painting is not an autograph Caravaggio. He has now published a beautifully illustrated book about the case, although unless you followed the story at the time and then rushed off to the Museum of the Order of St John (a lunchtime walk from the Inns of Court) to see the ‘Mahon Cardsharps’, you might not find it quite as interesting as I did.

The book contains more detail than the general reader might have wished. But it is interesting, and unusual, to see how a trial looks from the perspective of an expert witness, and if you are interested in how art experts approach questions of attribution, the book is full of insights. The rivalry between the opposing experts and their different disciplines is vividly described, and Professor Spear skilfully draws our eyes to subtle differences between the two paintings that a non-expert would overlook.

About half of the book is taken up with edited quotations from the transcript of the trial. This lends some drama to the narrative, although as courtroom dialogue goes, it doesn’t have quite the brio that a readership brought up on ‘Boston Legal’ or ‘Ally McBeal’ might have come to expect. Professor Spear recognises this problem. Describing Thwaytes’ counsel’s submissions, he says: ‘His opening occupied the rest of the day as he addressed issues in trying detail, often asking the Judge to read through documents she knew while he glossed on (sic) them. Perhaps such tedium is customary court procedure.’ Yes, well, we’ve all been there. There is nothing quite as tiresome as listening to one’s opponent’s submissions.

In the event, Rose J held that Sotheby’s had not been negligent, and dismissed Thwaytes’ claim. Reading Professor Spear’s book, it is almost impossible to see how the outcome could have been different; but the tale might have been a bit more gipping if the merits had been – or had been presented as being – more evenly balanced. After reading the book, you might like to go and have a look at the painting and make up your own mind.