Was it an explosion of nervous and artistic energy after his release from hospital, following the auto-amputation of his ear? Was it a frantic and tragic attempt to paint as much as he could before he lost the struggle with depression, which led to his botched suicide and slow death in July 1890?
In the past two years, several art journalists and scholars have revived another explanation for Van Gogh's extraordinary production during the final 10 weeks of his life in Auvers-sur-Oise, north-west of Paris: he didn't paint them all. Several of the Auvers paintings, they suggest, are fakes, painted, most probably, by Dr Paul Gachet, his doctor, sometime friend and the subject of two of the Auvers canvases.
The claims and counter-claims about the authenticity of at least four of the works (including one of the Gachet portraits) have already led to two court cases in France and a series of mutually insulting articles by some of the best-known names in Parisian art criticism.
An exhibition which opened at the Grand Palais in the capital this week claims to prove, scientifically, that all of the Auvers works are genuine and seeks to lay the controversy to rest. But there is small hope of that. The show has simply ignited another series of blasts and counter-blasts.
Supporters of the "Gachet fake" theory accuse the French museums service of mounting a self-serving exhibition, intended to whitewash (as it were) the doubts surrounding several Van Gogh canvases owned by the French state (as well as a couple of Cezannnes). The experts in the museums service dismiss the critics as amateurs, who refuse to allow scientific proof to disturb their pet theories, which they have erected from false intuitions and circumstantial evidence.
In other words, the show has become Exhibit A in a legal and art-political argument, as well as an exhibition. No matter. It is a fascinating show, as well as a fascinating argument and a fascinating story.
Dr Gachet was a railway doctor, a self-declared specialist in nervous problems, but also an amateur artist, who befriended several of the painters of the day (Cezanne, Pissarro, Monet, Renoir). It was Pissarro who recommended to Van Gogh's brother, Theo, that the troubled Vincent should be sent to live near Dr Gachet in Auvers after he emerged from hospital in May 1890.
The painter and the art-struck doctor got on well at first but Van Gogh began to have his doubts, writing to his brother that Gachet was "sicker than me. When the blind lead the blind, don't they both fall in the ditch?". This letter, and Gachet's unusual behaviour after Van Gogh shot himself - he failed to remove the bullet and, in effect, left him to die - have led some historians to blame the eccentric doctor for the artist's death.
Gachet assembled a large collection of works by his painter friends, including seven Van Goghs, three Cezannes, a Monet, a Renoir and several Pissarros. They were eventually donated to the French state by his son between 1949 and 1954. Almost all now belong to the Musee d'Orsay and almost all are in the Grand Palais exhibition. Alongside them hang other works by Van Gogh and Cezanne and many original paintings and self-declared copies carried out by both Dr Gachet and his son, under the pseudonyms Paul and Louis Van Ryssel.
But which are the Van Ryssels and which are the Van Goghs? The show has been put together by the chief curator at the Musee d'Orsay, Anne Distel, and Louis Van Tilborgh of the Van Gogh Foundation in Amsterdam. By hanging the Van Goghs and the Van Ryssels (Gachets) side by side, the curators hope to prove the first part of their argument. The disputed pictures - especially the "second" portrait of Dr Gachet - may be below the quality of the artist's best work but are infinitely superior to anything attempted by the doctor or his son.
Furthermore, the curators say, both the Van Goghs and the known Gachet paintings have been subjected to 12 months of the most minute and rigorous chemical and X-ray analysis. These investigations reveal that the amateur, and amateurish, Gachets always drew the outlines of their subjects and filled in the colours later, like a child painting by numbers. The disputed Van Goghs and Cezannes were not painted in this way. They were painted directly on to the canvas.
"The X-rays showed no signs of touching up. It showed a strong, firm line in Van Gogh's manner and a background filled in with large brushstrokes, crossing over one another, as in most of his paintings," reported Daniele Giraudy, head of the contemporary arts laboratory at the French museums directorate.
Convincing proof? Not in the least, say the doubters. "The star canvas of the exhibition [the portrait of Dr Gachet] is a cuckoo's egg," said Benoit Landais, the French art critic and Van Gogh specialist. He points to the letters sent by Van Gogh to his brother in June and July 1890, which gave detailed accounts of all his work in Auvers but made no mention of a second portrait of the doctor. M Landais says that the painting is manifestly a fake, probably copied from a photograph of the original.
Another French critic, Jean-Marie Tasset, says the exhibition shoots itself in the foot. By hanging two unimpeachable Van Goghs - The Church at Auvers and a self-portrait - close to the disputed painting, they have exposed the "second" Gachet portrait as "a lifeless, clumsy, soulless composition". On the contrary, says Mr Van Tilborgh of the Amsterdam Van Gogh Foundation, the portrait is a "moving work", well beyond the capacity of either of the Gachets...
And so the argument goes on. And on.
Along the way, the research by the French museums' laboratories has proved something which has long been suspected. Van Gogh used cheap materials in his Auvers period and some of the colours in his later canvases have faded or changed, especially those based on red. In that sense, all these later Van Goghs are "fakes": in the sense that they are not what the painter intended.
The foxgloves grasped in Dr Gachet's hand in the "first", undisputed,portrait have altered colour from mauve to blue. Curiously, exactly the same change has occurred in the second, disputed painting. If Dr Gachet faked this painting, did he use the same cheap paints as Van Gogh? This seems unlikely because in all the accepted Gachets, he is known to have used higher- quality materials. Is the "cuckoo's egg" a discoloured Van Gogh after all?
A Friend of Cezanne and Van Gogh, Doctor Gachet 1828-1909, at the Grand Palais until 26 April