In the Spring 2013 issue of The Journal of Art Crime, art theorist Thierry Lenain writes on "The question of the Value of Doubles in Autographic Arts".

This essay seeks to analyze the concept of the artistic value of copies, taking into account the comments of Renaissance, Early Modern, and Modern thinkers and artists, from Vasari to Friedlander. The essay is more philosophical/theoretical, rather than criminological, dealing with ideas rather than case studies. In the course of the essay, the reader is introduced to factions who praise skillful copies and others who dismiss any copy, no matter how skillful, out of hand as inherently worthless and bad. This overview shows the extent to which the treatment of the question of the double in painting has varied over time.

Isleworth Mona Lisa (Wiki)

'La Joconde' (1503-1506) has mostly hung in the Louvre (INV. 779) since Francois I acquired it in 1518.  Last week The Mona Lisa Foundation reintroduced the 'Isleworth Mona Lisa' which had not been seen in public for more than 40 years and declared that it was an earlier unfinished portrait of Lisa Gherardini del Giocondo by da Vinci.

Jamie Keaton for the Associated Press (published online here in Business Week) reported on the unveiling of the Isleworth Mona Lisa in Geneva on September 28.  Keaton points out the Alessandro Vezzosi (see below) 'declined to line up behind the foundation's claim that it was truly a "Mona Lisa" predecessor painted by da Vinci.  Here, on the Museo Ideale Leonardo da Vinci's website, Professor Vezzosi clarifies that research on  the "Isleworth Mona Lisa" will continue.

The Mona Lisa Foundation, a non-profit organization based in Zurich, has on its website a 22-minute video that walks the viewer through its claim, using historical documentation (including writings of 16th century art historian Giorgio Vasari), connoisseurship, critical comparisons and physical and scientific examinations.  Participants in the video include Professor Alessandro Vezzosi, Director of the Museo Ideale Leonardo da Vinci; Professor John F. Asmus, Research Physicist at the University of California in San Diego; Stanley B. Feldman, and art historian and principal author of "Mona Lisa - Leonardo's Earlier Version".  The video asks if it is possible that there was another Mona Lisa and if so, what could have happened to it? It is claimed that Giorgio Vasari and Agostino Vespucci mention a painting left unfinished.  In the early 20th century, Hugh Blaker, curator of The Holburne Museum  in Bath, believed in the two Mona Lisa painting theory and spent a decade looking for the unfinished version until he found it "in the Somerset home of an English nobleman" "who's family had owned the painting for nearly 150 years."  Blaker brought it to his London studio in Isleworth, then shipped it to the United States where it hung in the private offices of the Boston Museum of Fine Arts, according to the video.  Then in 1922, Blaker sent the Isleworth Mona Lisa to Italy for the opinion of experts there.  In 1926, Baker's stepfather, John Eyre, published "The Two Mona Lisas."  Henry F. Pulitzer, over a period of 26 years, liquidated his Kensington estate and part of his art collection to purchase the Isleworth Mona Lisa.  In 1979, after Pulitzer's death, the painting was locked up in a Swiss Bank vault "where it would remain for more than four decades."  "Can science at least succeed where connoisseurship has failed [in establishing the painting's authenticity]?" In 2004, the Isleworth Mona Lisa was removed from its security vault and "entrusted to world renowned art auctioneer David Feldman" (Vice President of the Mona Lisa Foundation).  "Over the next 12 years, the painting went through every test" including examination by Professor Asmus who has also examined the Louvre Mona Lisa who saying "Leonardo's hand" is evident in some aspects of the Isleworth Mona Lisa.

But there are more potent reasons to attach the greatest importance to the new discovery.  There is, in the collection of old master drawings at the Louvre an original pen drawing by Raphael, which is reproduced in Muntz's great work on Leonardo, and which is generally admitted to be a memory sketch by Raphael of Leonardo's "Mona Lisa."  Now this memory sketch is framed at both sides by two columns of which no trace is to be found in the Paris "Mona Lisa." These columns appear in the identical place in the Isleworth picture and are of immense value to the harmonious balance of the composition. 

In the notice sent out to the press it is stated that these columns are mentioned by Vasari, which is as little in accordance with facts as most of the other statements made.  Thus, one of the points quoted in favor of the authenticity of the picture is one of Leonardo's letters to Marshal de Chaumont.  In this letter occurs the passage: "E portar con mecho due quadri di due Notro Donne di varie grandezze le qual son fatte pel cristianissimo notro re."  While most art historians have misread this to mean that Leonardo took with him "two pictures of Our Lady, of different sizes," the writer of the widely circulated notice says that the existence of two versions of the "Mona Lisa" is proved by Leonardo himself referring to two portraits.  A literal translation of the quoted passage would however run as follows: "And take with me two portraits of two of our ladies, of different sizes, which have been painted for our most Christian King", the letter thus reflecting clearly to two different ladies and not to two versions of the same. 

However, no specious arguments are needed for the Isleworth picture, the quality of which may speak for itself.  And a close investigation of the picture leaves the firm conviction that, though not altogether from the hand of Leonardo da Vinci himself, it emanates most certainly from his studio and was very largely worked up by the master himself.  The hands, with their careful and somewhat hard drawing and terra cotta coloring, suggest at once the name of Leonardo's pupil, Marco d'Oggionno, whereas the inimitably soft and lovely painting of the head and bust, the exquisite subtlety of the expression, the golden glow of the general coloring, can be due only to Leonardo.  The face shows none of the defects of the Louvre picture, which are probably due to clumsy repainting. 

The present owner of the picture acquired his treasure only about a year ago.  He found it hidden in a Somerset mansion where it had been for a century and a half, and whither it had been brought from Italy.