The Madonna of the Yarnwinder has been the subject of some debate among art experts.
Their questions centre on just how big a contribution Leonardo da Vinci actually made to its composition.
It is a discussion to which there is unlikely to be a definitive answer any time soon.
Nonetheless, what is known about the painting, which was central to an extortion case at the High Court in Edinburgh, remains highly intriguing.
In 1500 Leonardo had returned to Florence, where he set about working to re-establish his artistic career.
Some of the best evidence of his activities at the time comes from correspondence written by one of the Renaissance's most influential female figures, Isabella d'Este.
In one letter from her representative in Florence, Fra Pietro da Novellara, mention is made of Leonardo's obsession with geometry.
It describes how his pupils were making copies of his paintings, to which he occasionally put his hand.
The letters also describe two works by Leonardo.
One was for Florimond Robertet, the secretary to the French king - it was a piece which later became known as the Madonna of the Yarnwinder.
Grove Art Online, an art encyclopedia, notes that the best versions of the "much copied" painting are in the Duke of Buccleuch's collection and a New York collection.
It states: "It is difficult to assign a single, finished, wholly autographed painting to the years 1500 to 1508.
"It is reasonable to assume that the Madonna of the Yarnwinder was completed, but even that might not have been entirely by Leonardo."
Regardless of that fact, most critics agree that the great Renaissance artist played some part in creating the work.
And that, in itself, is enough to give it enormous value.