Two islands in the Seine remain strikingly apart from the rest of the city. Gully Wells finds out what Balzac meant when he talked about "gastronomy of the eye"

Paris was conceived and born in the murky, fast-flowing waters of the River Seine. More than two thousand years ago, the Parisii—who seem to have spent their time messing about in boats, fishing, and trading with other Celtic tribes—began to settle on an island, the largest in a small archipelago in the middle of the Seine. Surrounded, nourished, and protected by the amniotic fluid of the mother river, the island must have appealed to these nomadic, almost amphibian, people as a relatively safe place to put down their first tentative roots. The same idea struck the Romans, who, under Julius Caesar, came, saw, and conquered in 54 b.c., and ever since then, the island, which we know as Île de la Cité, has been geographically, historically, and emotionally the very heart of Paris.

What had been a group of maybe ten islands surrounded by marshes coalesced over the centuries into the two iconic but completely different islands that lie at the center of the city today. Île de la Cité is obviously the much older, more serious, sibling. From the very beginning, both secular and religious power were based here—represented by the ruler's palace, on the site of what is now the Conciergerie, and Notre-Dame, respectively. Bridges were built, first by the Romans and then by subsequent kings, and the city gradually spread to the right and left banks of the Seine.

But it wasn't until much later, in the early 1600s, that Île de la Cité's irresistibly beautiful, sometimes frivolous, yet always seductive younger sister (how could Île St-Louis be anything but feminine?) was born. Her father, King Henry IV, finished building the Pont-Neuf (his handsome, smiling equestrian statue stands at its midpoint, on the western tip of Île de la Cité) and then decided to join together two small uninhabited islands to the east in order to create Île St-Louis. Instead of evolving slowly and messily over time, the new island was developed with astonishing speed, between 1614 and 1664. With Cartesian precision, it was sliced into symmetrical lots, the rue St-Louis-en-l'Île was built straight down the middle, and a series of honey-colored hôtels particuliers were constructed along the quais, in the newly fashionable classical style.

Chances are that if you've been to Paris, you've been to Notre-Dame. And afterward, you may have walked across the little bridge that connects Île de la Cité to Île St-Louis, and stopped off at Berthillon for an ice cream—but not in August, when the shop is (bizarrely) closed for the fermeture annuelle—before taking in a couple of galleries or wandering into the Church of St-Louis-en-l'Île. Which was just about the extent of my knowledge of the two islands, even though I was born on one and used to stay with my godmother quite regularly on the other. So the last time I found myself in Paris, I decided that I would do better than that. I'd do some research, dig around a bit, walk a lot, and spend time talking to the people who live and work on the islands. Above all, I would not hurry. In other words, for one week I would be a flâneuse des îles. Now flâneur is not an easy word to translate. Balzac probably defined it best when he wrote, "Ah, to wander in Paris! Adorable and delectable existence. Flânerie is a science, it is gastronomy of the eye."