The Mona Lisa is not the only enigmatic painting in art history. The splendid exhibition of portraits by the great Spanish artist Francisco Goya (1746-1828) at the National Gallery in London revives another art historical mystery.
Goya did about 160 portraits in his long career and 67 of them are in this splendid exhibition, which I visited in London. He was an acute observer of society - good, bad and ugly - and he showed personalities with incomparable skill.
He became First Court Painter to the Spanish Crown so a number of the portraits are of Spanish royalty and aristocratic courtiers.
These are done with all the flourish of uniforms, medals, jewellery and high fashion. Then there are less formal images of friends, poets and painters as well as his striking self-portraits.
The enigma lies among the aristocrats, particularly Goya's relationship with the Duchess of Alba, Maria del Pilar Teresa Cayetana de Silva Alvarez de Toledo y Silva Bazan, long rumoured to have been his mistress.
She was the sole heiress to the highest ranking duchy in Spain, one of the largest landholders in Spain, enormously rich and famous for her independent mind and lively personality. She was married, aged 12, to an older cousin.
(Goya's portrait of her husband, who took her name as the Duke of Alba, is also in the exhibition. He was a cultivated and athletic man, particularly interested in music. Goya painted him leaning on a piano studying a score by Hayden. He is wearing boots and spurs and has just been riding.)
Goya met the Duchess before 1794 and became friendly with her household. In a letter to a friend, he describes the Duchess bursting into his studio and demanding that he paint her make-up for a party she was to attend. This is in keeping with her reputation as a wild and free spirit.
Goya painted two full-length portraits of her; both are rarely put on show. The first shows her in a white dress with a red sash and a red ribbon setting off her striking masses of dark hair. She stands, slim, haughty, aristocratic and very beautiful on sandy ground in a wide landscape. Goya's signature and dedication, 'to the Duchess of Alba, Fr.co de Goya', is written in the sand. This painting is still in the Alba family collection in their palace in Madrid.
It is the second portrait on show in this exhibition. In the work, the Duchess is dressed in black in traditional Spanish dress and mantilla. Goya was in Andalusia in the south of Spain at this time as part of the entourage of the Duchess. The Duke had died suddenly, aged 39, in 1796 in Seville.
Goya drew in notebooks he carried and his drawings of the Duchess' household are very intimate. In 1797 she made a will which left a substantial income to Goya's son, Javier. The painter had just recovered from an almost fatal illness that had left him totally deaf.
That same year, he signed and dated this final portrait of the Duchess. She stands in a landscape similar to the first portrait. Her beauty is as striking as ever and she points again to something written in the sand. It says, "Solo Goya" - "Only Goya" and on her right hand are two rings. One says, "Alba" and the other, "Goya".
Is she referring to Goya as an incomparable painter or as a very close friend, perhaps a lover? Is it possible that the most distinguished member of the Spanish aristocracy could have taken an ageing, sick, deaf, social inferior to her bed even if she recognised his genius as a painter?
Or is she just indicating that he was the best painter in Spain? Or is it Goya's own declaration of his superiority as an artist? Are the rings authentic or were they added by a later hand?
Manuela B. Mena Marques, who wrote the introduction to the comprehensive catalogue that accompanies the exhibition, dismisses the idea of a liaison as an "absurd legend", yet it persists.
Goya took the painting back to Madrid where it stayed in his studio and was still listed there in an inventory taken in 1812 when his wife died. It has been loaned to the exhibition by the Hispanic Society of New York, which regards the portrait as a jewel in its collection.
This grand exhibition also includes the large paintings of the Spanish royal family on loan from the Prado in Madrid. These paintings, especially the huge painting of The Family of Charles IV with a dozen or more members of the family, have always astonished by the direct honesty of the portrayals in all their ugliness and pomposity.
Across the room from the famous Duchess is a portrait of the Spanish Queen in the same costume and in a very similar pose and setting.
The Queen has a certain dignity but is unprepossessing. The Duchess and the Queen were rivals especially in fashion but royalty could not match the vivid personality of the Duchess. If it was a competition there is no doubt of the winner.