A picture of Paris

On the street below my window is one of those metal lollipops used in Paris to mark sites of historical interest. The Tambour Royal once stood here, an 18th-century inn that was the haunt of tarts, artists and similar low life. Outside the city's toll gates - beyond the pale - the Tambour was famed for its pleasure gardens, a lesser version of the ones at Vauxhall. Anything could happen here and most things did. Beaumarchais mentions the gardens in The Marriage of Figaro as a place of clandestine meetings.

What he doesn't mention, though, is swings. This bit of Paris was the original swinging city: citizens of all ages and types yawing back and forth on banks ofbalançoires, or their grown-up version, escarpolettes. If you were of a thoughtful turn of mind, the rocking motion of these might strike you as suggestive, the kind of thing that would probably Lead To No Good. Priests denounced swings from the pulpit. In 1767, Jean-Honoré Fragonard, a Tambour regular, got his own back by painting a picture of a pretty young girl being swung so hard that - horreur! - her slipper had flown off. The man pushing her is a priest. The picture is called Les hasards heureux de l'escarpolette, better known to us as The Swing.

The Swing may be in London's Wallace Collection but Fragonard is still here in Parmentier, flitting irreverently down the rue St-Maur. Like any major city, Paris is full of art, but unlike most cities, it is also full of art ghosts. You can't walk across the Place de la Concorde without seeing Jacques-Louis David, pencil in venal hand, sketching Marie-Antoinette on her way to the scaffold. Flee him into the nearby park and you'll find Manet's Music in the Tuileries Gardens in full swing: a work also allegedly in London - in the National Gallery this time - although it's really here under these dappled trees, with Manet's stiff French matrons sitting on the same stiff French chairs.

It was the poet Charles Baudelaire who ordered Parisian artists to stop painting history and start painting what they saw, namely modern life. The new artist, said Baudelaire, should be a flâneur: a dawdler on street corners, a hanger-about. The invention of plate glass had made Paris the world capital of window shopping; Baron Haussmann's new boulevards opened up vast vistas, allowed a new kind of cityscape. Parisians from the 1840s saw differently from other people, and gave more time to seeing. In turn, they bought paintings by Monet and Manet and Caillebotte and Sisley, which reflected back their own modern gaze, their new-found fascination with the art of looking.

And that's what makes Paris the best place in the world for seeing art. It isn't just the Louvre and the Musée d'Orsay, it's the places around them: Manet's parks, Monet's Rue Saint-Denis, Caillebotte's bridge over the tracks at the Gare St-Lazare. There's a different way of seeing here, a different gaze. And August, when the city is still and empty and the streets are filled with artistic ghosts, is the best time of all to flâne.

As I speak, one of these ghosts is stalking the Hôtel Salé, just down the hill in the Marais. The Musée Picasso is preparing its autumn show, X-Rayed from Picasso, which exposes for the first time the inner life - the image intérieure - of the Spaniard's ceramic sculptures.

These have always seemed curious objects, although no one guessed just how curious until a French photographer called Xavier Lucchesi recently X-rayed them. What Lucchesi discovered was that Picasso had filled works such as The Goat and The Monkey and its Baby with secrets. Entombed in the clay of the first is a bicycle chain, shards of pottery and a wicker basket handle. Each sculpture is a time capsule, set to be opened only after Picasso had gone to wherever it is that good artists go when they die: heaven or, more likely, La Coupole. Lucchesi's X-rays, with their auras and spectral treasures, are part of the ghost story that is August in Paris.

From the Hôtel Salé, you can stroll through empty streets to another exhumation: of Monet this time, or rather of his famous Nymphéas - the eight panels of waterlilies at the Orangerie, open to the public for the first time in seven years. When the museum closed its doors in 1999, half a million visitors a year were surging through its oval galleries. Queues doubled back on themselves for a kilometre. Yet now, in August, I stand in line for five minutes to see the newly reopened Orangerie, its 91 metres of Monets.

Resurrection isn't too strong a word for what has gone on in the Tuileries. If something of the Orangerie's ramshackle charm has been lost, what has been gained is the natural light Monet meant his lilies to be seen by. This was sacrificed when the oval galleries' glass roof was built over in the 1960s. That extension has now been demolished and the limpid light of Giverny, or at least its urban cousin, once again shines on Monet's masterwork. Come to see it in October and you will find yourself pushed along by ranks of respectful French schoolchildren. Come to Paris before this year's rentrée and, if you get there early enough, you may have the waterlilies to yourself.

Eurostar (08705 186 186; eurostar.com) offers return fares from £79 in August

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