ART / A dance to the music of time

`Unless you are mad, stupid, terminally narrow-minded or have some othe r such watertight excuse, make sure you visit it.' Andrew Graham-Dixon falls in love with Nicolas Poussin

The young man at the centre of the procession has brought something very unusual with him, something he wants to share with everyone present. It is a huge decapitated head on a pole. He holds it up for all to see like an athlete holding up a trophy as he runs a lap of honour. Trumpeters blowing a fanfare precede him, while the people in the crowd exhibit what can only be called the full gamut of human response. Some hold their hands up in horror and relief. Some thank the lord. Some shake their fists. Some embrace one another, or raise their arms in acclaim like sports fans celebrating a score (yesss!!!) by their team. Some wonder what on earth is happening andneed to have it explained to them. Some debate the morality of it all. Some, babies and young children, laugh or cry or just burble and carry on doing not very much in particular. Some think the young man is rather attractive.

This is David shortly after slaying Goliath as painted by Nicolas Poussin, circa 1632. On loan from the Dulwich Art Gallery, it is one of more than 80 paintings in "Nicolas Poussin: The Father of French Painting" at the Royal Academy, which is much too remarkable and cornucopian an exhibition for mere recommendations. Unless you are mad, stupid, terminally narrow-minded or have some other such watertight excuse, make sure you visit it.

The Triumph of David is worth dwelling on because it is one of those masterpieces into which a great painter appears to have squeezed almost every part of himself. An ancient myth, reimagined, has mutated into an entire and thronged world. Poussin's viewof the world in general is also implicit within it: generous, excitable, perceptive, easily roused to sensual enthusiasm (the colours in the painting are as alive as the people) as well as more than a little morbid.

Poussin's David is not Michelangelo's boy giant, nor is he Donatello's self-conscious and flirtatious teenager. He is ordinary. He is fit and healthy, slightly chubby, too, and not particularly intelligent. His skin (flushed pink by his exertions) is made to seem still pinker and more rudely healthy by the nasty green that has begun to tinge the head on the stick. Goliath, who looks just like David but older and bearded, chuckles in death, as if he might have died seeing a part of himself in his nemesis.

This great image of a victory for the forces of light contains Poussin's most profoundly felt and painfully acquired piece of wisdom: the knowledge that light itself easily turns to darkness and that bright-faced young Davids are always turning into Goliaths, just as day is always turning into night. The crowd in the painting is endlessly fascinating. But its fervency is also slightly repellent.

Metamorphosis was Poussin's great theme: change for better or worse, but always change. Many of his paintings are set at dawn or dusk, when light is draining in or out of the world: moments when the world's eternal changeability is both most manifest andmost poignant. In his first and most bloodily crepuscular pictures, Poussin confronts change like a man still so moved by it, so profoundly affected by its inevitability, that he can hardly bear to face it.

Echo turns amazingly and terrifyingly to stone while Narcissus indifferently begins to undergo his own transformation: his hair has begun to become flower stems and blossom.

Two shepherds and a shepherdess in Arcadia discover that death exists even in paradise, by bending over to read the inscription on a tomb: the composition of this picture is structured around the declining shape their three bodies make, the acquisition of knowledge painted as a fall.

The first painting seen in the exhibition is one of Poussin's most violent pictures of change, in this case the change wrought on one man's body by other men. A saint, pinioned face-up and naked over a workbench, is having his guts meticulously removed. The executioner, who has made a neat incision in the victim's body just below the breastbone, is gently easing out the intestines. He does it with the care of a butcher used to handling strings of sausages without rupturing the skin. The gentleness of his inhumanity is horrifying. Behind him an assistant turns the large wooden roller to which the pink strings have been attached. This misery will be spun out. An old man draped in white, some pagan high priest, points to the obscure brazen god on whose behalf this punitive act is being carried out, but the victim is too far gone to care.

Nobility, heroism, in fact all good or grand things are always on the verge, in Poussin, of turning into their opposites. He lived all his mature life in Rome - the only city in the world rich enough to sustain his imagination - but he didn't sentimentalise the place or its history. He painted its mythical origins in his The Rape of the Sabines. A crowned man stands in elegant profile on the steps of a temple before a plaza, and incites his men to carry off every woman they can find. His soldiers scurryabout grabbing women; the women bite and scratch. The crowned man is Romulus, the soldiers are the first ancient Romans and the women belong to the tribe of the Sabines, but it is best not to know too much about Poussin's subject matter. This is an image of abduction and pillage and rape as it has always been and will always be.

No wonder the greatest painter of the French Revolution, Jacques Louis David - a man who lived his life at the eye of the storm, who saw the power of crowds turn so rapidly from good to evil, who saw idealism turn to dust and victory turn to defeat - wasenthralled by Poussin. Among other things, he learned from him the tremendous poignancy that a still-life detail can impart to a narrative painting. The heap of useless armour that Poussin painted under the dead man's bed in The Death of Germanicus, a bright reminder of triumphs past and never to be re-enacted, like a pair of football boots hung up never to be used again - this is a brilliant invention, which predicts the useless quill which David placed in the dying Marat's limp grasp, and the sewing basket on the table in his painting of Brutus and his wife grieving over their dead sons.

Poussin's influence on the artists who came after him, particularly the French artists, had been immense. He was one of the first great narrative painters to invent his own subjects rather than paint to commission. He invented himself and to a great extent he may be said to have invented French painting. The storms and deluges of his last years anticipate the tarnished copper seats and leaden skies of Theodore Gericault. His voluptuous, statuesque women, the soft but chiselled dancers and orgiasts of his bacchanalia would preoccupy the imagination of Ingres for more than 50 years. His vivid dancing colours, bright shapes laid on dark grounds as freely as the elements in a collage, must have been at the back (or the front) of Matisse's mind when he invented the great jazzy compositions of his last years.

But Poussin's formal inventiveness, so fruitful for later paintings, had nothing of the spirit of formalism about it. He painted as only a great and perceptive voyeur, living in a turbulent century, could paint. In 1649, commenting on the beheading of England's unlikely Goliath, Charles I, Poussin wrote to a friend, "It is a great pleasure to live in a century in which such great events take place, provided one can take shelter in some little corner and watch the play in comfort." His painting fed off the world but it was also his way of making sense of the world, of distancing himself from it, of producing order out of its ceaseless flux.

Perhaps, too, it was one of his ways of disciplining himself. This great moraliser of change seems to have been a most changeable man. In his youth, it was said, he was a boisterous, rebellious character, a street brawler and an ardent, hopeless lover. In his maturity he became well-known for his asceticism and his adherence to the strict, temperate, resigned philosophy of the Stoics. His painting style also changed. His Venetian melancholia became Roman fortitude as he stopped painting like Titian and started painting like Raphael. Colour became line. Feeling became reason. Instinct became geometry. That, at least, is how one theory of Poussin's development goes. He contributed to it himself, declaring in a letter to a friend, "My nature compels me toseek and love things that are well ordered, fleeing confusion, which is as contrary and inimical to me as is day to the deepest night."

This is one of the older Poussin's most well-known statements, but it is not entirely truthful; his nature was more humane, inventive and generous than the remark implies. The kinds of structure and order which he devises are open, not closed, and he is constantly breaking with decorum in ways that are more often associated with other and more patently anarchic geniuses like Rembrandt or Shakespeare.

A tremendously bold and affecting instance of this occurs in the picture of Extreme Unction, which he painted as part of his first series devoted to the subject of the Seven Sacraments. The dying man and those watching him die absorb the viewer's attention, but not entirely. To the extreme right of the picture a beautiful girl is running out of the room. She is running, and looking straight at us, in a way that makes it quite plain that she is going to meet her lover. Death often leads people to think of sex, which is natural enough, if not something polite people notice. But Poussin saw the world clearly enough, and accepted what he saw there sufficiently, to paint one of the few masterpieces - maybe the only one - to observe this.

His unruliness was inseparable from his love of life and his Stoicism cannot have been at all the conventional kind: a check placed on his enormous appetites and humanity, but one that could not restrain or mute it.

Poussin never stopped exploding until he died and he kept reinventing himself in ways that only modern artists are, by some, supposed to do. Painting a Lamentation in 1658, he displaced all the feeling in the tragic subject out of the figures and into the colours. Purple and red and pink and blue shriek a discord that makes the picture as effective seen from 50 yards away through half-closed eyes as it is from close to.

In his valedictory late landscapes, Poussin invented Surrealism (making the invention of it, in fact, unnecessary). He painted among other things an odd dream of a giant, guided by a man standing on his shoulder like a parrot, walking through beautiful countryside past a woman standing in clouds that looked like backlit coal smoke. His hand shook terribly as he painted, but Poussin worked with the constraint and made the handicap of an old man into a form of pictorial originality: a distant dreamed blurriness. Sir Joshua Reynolds, of all people, owned this picture, the Landscape with Orion. It must have been a perpetual reproof to him.

Poussin died in 1665, having been forced at last to put away his brushes, as Germanicus had had to put away his armour. Perhaps he kept them under the bed. Almost his last recorded remark was his definition of painting as "an imitation in lines and colours of all that is under the sun".

n For details, see listings below