Van Gogh only got one major review in his career, and he was mystified by it. When the critic Albert Aurier described his six paintings in the 1890 Brussels exhibition of Les XX as the product of a 'terrible and distraught genius', the artist responded that, far from being a genius, he was 'very secondary' and that his sunflowers - now in the National Gallery - were no different 'from so many pictures of flowers more skilfully painted'. If he were alive today he would probably have protested at the National Gallery making an exhibition of his work the high point of its bicentenary programme, but he would have liked its focus on his inspirations rather than his distress.

The show, which covers the artist's period in Provence from February 1888 to May 1890, takes its subtitle, Poets and Lovers, from a pair of portraits painted in Arles in 1888. 'The Poet' was the dreamy Eugène Boch, a young Belgian painter with a 'Dante-like' head; 'The Lover' was the handsome Zouave lieutenant Paul-Eugène Milliet, whose success with women Van Gogh envied while consoling himself that 'he has all the Arlésiennes he wants' but 'can't paint them'. In the opening room the two portraits flank 'The Poet's Garden' (1888) showing a couple walking hand in hand in the public park opposite the Yellow House he planned as a base for his projected 'studio of the south'. Van Gogh admitted that the park was 'nothing special', but after reading about Boccaccio and Petrarch - who met his muse Laura in nearby Avignon - he imagined it haunted by the spirits of Italian early Renaissance poets and embarked on a series of paintings of lovers strolling in its gardens, intending them as decorations for the guest room he had earmarked for Gauguin.

Van Gogh's early views of Arles thrum with expectancy. In the famous painting of 'The Yellow House' (1888) the streets are illuminated like an opera set, with members of the chorus milling around waiting for the entrance of the principal singers. The long-awaited principal, Gauguin, would arrive in October, but he is strangely absent from this exhibition. The only paintings dating from his fateful two-month stay are 'Van Gogh's Chair', 'The Sower' and an uncharacteristically broad-brush view of 'The Alyscamps' cemetery (all 1888), painted in his company. Everything else was painted in anticipation of his arrival or in recovery from the aftermath of his visit. This is not the Vincent and Paul show: its preoccupations, and style, are entirely Vincent's. Van Gogh was drawn to the south not by Gauguin but by the now half-forgotten Provençal painter Adolphe Monticelli, whose fiery palette and juicy impasto embodied 'the rich colour and rich sun of the glorious south' to his northern eye. It was the example of Monticelli, who 'did the south all in yellow, all in orange, all in sulphur' that emboldened him to paint the yellow on yellow still life of sunflowers which astonished Gauguin. 'Merde! Merde!' he remembered its impact years later. 'Everything is yellow! I don't know what painting is any longer!'

Van Gogh associated sunflowers with gratitude, but not all the paintings in this show have poetic messages. Urged to work from imagination by Gauguin, he was happiest when grounded in reality. He beat the bounds of Arles looking for views that lent themselves to dynamic compositions à la Japonaise: with its diagonal tree trunk, 'The Sower' owes as much to Hiroshige as to Millet.

A series of large reed pen drawings made around the ruins of Montmajour Abbey deploy the graphic language of dots and dashes of the Japanese woodcut. A panoramic view of the plain of La Crau - 'one of the best I've done with my pen' - testifies to Van Gogh's love of observational detail, from the piled trucks of the passing goods train in the middle distance to the pebbles and tufts of dry grass at his feet. This marvellous set of drawings required 50 sessions, battling heat, mosquitoes and the Mistral: 'If a view makes one forget those little vexations,' he reasoned, 'there must be something in it.'

For Van Gogh, working en plein air was a tonic; in the asylum at Saint-Rémy it became a form of therapy. Back in the hospital garden in April 1889 after a breakdown, he reported to his brother Theo, 'I recovered all my clarity for work', but hospital regulations obliged him to finish works begun outdoors in the studio. Without the reality check of an actual view, his increasingly calligraphic brushstrokes went their own way: the treetops in 'A Wheatfield, with Cypresses' (1889) lick the sky like flames and the clouds form themselves into ectoplasmic arabesques unknown to meteorology. In 'The Olive Trees' (1889), the billowing clouds, mountains, treetops and earth make you feel seasick. In the greatest of Van Gogh's Provençal landscapes, observation and imagination are in perfect balance; when the balance of his mind is upset you can read it in the swirls of paint, like reading tea leaves.