For Lesley Stevenson, it was just another day at the office.

But then, as the conservator X-rayed paintings ahead of a new exhibition at the National Galleries of Scotland, something extraordinary happened. 

Vincent van Gogh himself was staring at her, his face emerging from the back of another painting. 

Hidden behind Head of a Peasant Woman was an incredible self-portrait of a bearded man in a brimmed hat. He looks intently at the viewer, the right side of his face in shadow.

For now, the X-ray is the only proof of the hidden portrait. Researchers are still working out how to remove the glue and cardboard from the painting without damaging Head of a Peasant Woman. But Stevenson is still "hugely excited".

"This is a significant discovery because it adds to what we already know about van Gogh's life."

There is no doubt: there is a huge appetite from art lovers all over the world to witness the works of Vincent van Gogh. Millions flock to museums to see his art, from masterpieces like The Starry Night to relatively unknown sketches.

In his own lifetime, van Gogh's work was overlookedHe sold very few paintings, often trading them instead for food or art supplies. . But by 1947, when a huge Vincent van Gogh show opened in London, his art was so popular that the crowds wore through the floorboards in just five weeksWhen the show was over, the museum wrote to the Arts Council asking for money to fix the three years' worth of damage that had been done to the floors while the exhibition was open..

Britain had just won the Second World War, but it had been plunged into an even harsher period of austerityWhen the government cuts down on public spending. . People were "colour-starved," as one newspaper put it.

In the 21st Century, the fans flocking to see Van Gogh's work are not content with simply viewing his paintings. They want to know more about the man behind the portraits, and where his extraordinary genius came from. 

Van Gogh's life story is familiar to many. In the last decade of his life, plagued by depressionLow mood that affects someone's daily life, and can last for weeks or months. and mental illness, he cut off his own earThis was cut off on December 23rd 1888, about 18 months before Van Gogh died. For more, read the related article below., spent time in an asylum and eventually committed suicide aged 37. 

For years, critics obsessed over how the tragedy of his life influenced his art. In 1910, an Observer critic called some of his paintings "merely the ravings of a maniac".

Art historians have long discussed the idea that there is a link between creativity and mental illness. It has been believed so since the Ancient Greeks: "There is no great genius without some touch of madness," wrote AristotleA student of Plato, tutor to Alexander the Great and the father of political philosophy.. 

Great artists like Lord Byron, Virginia Woolf and Van Gogh all reinforced the idea. Perhaps the extreme emotions and thinking "outside the box" all help to aid creativity.

But Van Gogh's letters show that often he did not paint at all during his darkest periods. In fact, it was the opposite: painting was a way to try to keep sane.

There are plenty of great artists who did not suffer from mental illness. Many who do find that it stops them working at all, and today art is often used as a form of therapy. Van Gogh's sunflowers, one of his most famous series, was painted in a happy period. 

Now long dead, the artist himself cannot shed light on this debate. For now, the question remains: was he great because of his problems - or in spite of them?

Yes: Van Gogh wrote to his brother Theo that he "made a point of trying to express sadness, extreme loneliness" in his paintings. His art told the viewer "what I can't say in words". 

No: This is a dangerous stereotype. Van Gogh's genius was not in his mental illness or emotional state, it was in the way he looked at nature and the artistic choices he made on the canvas.

Or... Millions of people around the world suffer from mental health conditions such as depression. Few go on to be world renowned painters. Geniuses like Van Gogh simply have more publicity.

Overlooked - He sold very few paintings, often trading them instead for food or art supplies. 

Five weeks - When the show was over, the museum wrote to the Arts Council asking for money to fix the three years' worth of damage that had been done to the floors while the exhibition was open.

Austerity - When the government cuts down on public spending. 

Depression - Low mood that affects someone's daily life, and can last for weeks or months.

Ear - This was cut off on December 23rd 1888, about 18 months before Van Gogh died. For more, read the related article below.

Aristotle - A student of Plato, tutor to Alexander the Great and the father of political philosophy.