The descent into madness led to the creative flowering of one of art's supreme geniuses

It was on Christmas Eve in 1888 that Vincent van Gogh, exhausted physically and emotionally after quarrelling with Paul Gauguin, his friend and fellow painter, snapped under the strain and cut off the lower half of his own left ear.

From that point onwards, his was a spiral descent into madness, but with it came the greatest flowering of a creativity which has led him to be regarded as the greatest Dutch painter since Rembrandt and one of the formative influences on the development of modern painting.

He entered an asylum but in 1890 he was deemed well enough to leave the mental institution and move to Auvers-sur-Oise, which saw the greatest outburst of his genius. He painted 70 pictures in 70 days.

His portraits there were almost all of young women. He had only one male sitter, his physician Dr Paul Gachet, a man who evaded his own loneliness and melancholy by immersing himself in his work. Van Gogh described him as "sicker than I am". The paintings of Gachet are regarded as among the most significant in modern Western portraiture.

Towards the end, Van Gogh painted two studies of girls, one of them Young Peasant Woman with Straw Hat Sitting in the Wheat in which a young girl, in a pale gown and yellow straw hat, was set against a green wheat field. There was a deep mystical communion about it, with nature as a regenerative force. But there were poppies among the grass, the colour of blood.

There was ambiguity too in his other paintings of that final period, with death and timelessness expressed in the cypresses, olive trees, and the night sky with its stars.

Van Gogh wrote eight letters to his family in that final month, but they gave no clear indication of which canvas was his final work. It has long been supposed that his last painting was Crows over the Wheat Fields. And it would be apt if that were so. There is an apocalyptic quality to it with the intensity of its darkening blue sky, its hovering flock of black crows, and its disorienting perspective. It seems to mark a return to van Gogh's early years as a Christian missionary, speaking of the Crucifixion or the Last Judgment.

Yet there is another canvas, Daubigny's Garden, as well as a study of some old thatched roofs mentioned in a letter to his brother Theo.

Four days before he died, van Gogh wrote of two sketches of vast fields of wheat after the rain. One of these might well be Ears of Wheat, a close-up of "nothing but ears of wheat with green-blue stalks, long leaves like ribbons of green shot with pink, ears that are just turning yellow".

Or it might be the painting Wheat Field under Clouded Skies, in which the bounty of a landscape of cultivated crops moves wildly in the wind beneath a darkened, louring sky.

"I believe these paintings say what words cannot," he wrote in his final letter. "There are so many things I would like to write to you, but I feel it is futile."

The letter was not sent. It was found on his body. He died among these very fields. He left his easel against a haystack and shot himself with a revolver. The bullet passed beneath his heart, and though he fell, he rose again, three times, and returned home without telling anyone about his injury. The next evening he collapsed. When Dr Gachet told him that he still hoped to save him, the painter responded: "Then it will have to be done over again." His final words, to his loving brother Theo, were : "La tristesse durera toujours" – the sadness will last forever.