Can a single edifice define a city? Notre Dame cathedral may well be the one truly definitive monument in the world.

Even as images of the devastating fire began to circulate, commentators were pointing out that Notre Dame stands at the dead centre of historic Paris. But that geographic reality is only one way of measuring Notre Dame’s essential role in defining the city.

For residents and tourists alike, Paris is a beloved visual landscape, one that’s synonymous with the city by the Seine. For centuries, Notre Dame, and its spire in particular, have been at the absolute centre of that visual landscape.

The cathedral is the essence of what Paris means to all those who consider it the most beautiful city in the world, both those who walk the streets of Paris daily and those who visit it only once — or perhaps have seen it only in their dreams.

For believers and non-believers alike, Notre Dame has always been as essential to the reality of Paris, the lived city, as it is to the city’s myth. And Notre Dame is the only edifice in Paris that has played both these roles.

Before travel was easy, people knew foreign cities only through city views, paintings and engravings that portrayed the great cities of the world as vast panoramas displaying their attractions. In such views, many cities are virtually indistinguishable; few landmarks stand out from the mass of construction and seem unique to any one city.

Notre Dame was always one of those exceptional landmarks. Every early cityscape features its towers — and above all its spire — looming large above the city’s built fabric. Paris is known today as one of the world’s greatest walking cities, a pedestrian’s dream. Tourists are encouraged to wander through its neighbourhoods and visit its famous monuments on foot, and the city is compact enough to make this easy to do. It’s also a great walking city for its residents.

For those lucky enough to live in central Paris, it’s possible to cover the distance between even two widely separated neighbourhoods on foot in a reasonable amount of time.

On Sundays and holidays, the city becomes the perfect place to take advantage of leisure time with the same long walks recommended by every guidebook.

For all these Parisian pedestrians, Notre Dame remains, or at least it remained until Monday, a defining monument in another, very practical way, a way their precursors through the centuries would have understood.

As you walk through so much of central Paris, the spire of Notre Dame is visible along the way, one of the few telltale markers that dominates the cityscape and tells you where you are.

I am only a part-time Parisian. When I’m in Paris, I try to walk almost everywhere. I flew out of the city most recently only the day before clouds of smoke billowed out of Notre Dame.

As I thought back over my last days there, I realised how often I walked near Notre Dame. I realised, in particular, how often I looked up from the street-level beauty that is so real in Paris to take in a glimpse of Notre Dame’s spire. I also realised how meaningful the view of that spire has always been for me.

Of course, as French President Emmanuel Macron quickly promised, “we will rebuild”. The cathedral will one day be open again to floods of tourists.

The time will come when Parisians will be able to walk through the Île de la Cité and no longer think only of what may well become known as “the Great Fire” and the day when Notre Dame was engulfed in flames.

In time, a majestic spire will surely rise atop Notre Dame again — even though today it’s hard to imagine how that feat will be accomplished.

People may even learn to live with the ersatz and to forget the images of destruction. For now, however, and for the conceivable future, the panorama of Paris will be defined by an enormous absence, that of Notre Dame’s sublime spire.

The question is open: is it possible to rebuild an iconic monument? An essential part of Paris’s essence disappeared on Monday. When I next return, will that destruction have redefined the city? How much of the magic that makes Paris Paris will have disappeared along with the great cathedral’s iconic spire?

• Joan DeJean is Trustee Professor at the University of Pennsylvania. She is the author of 11 books on French literature, history and material culture, including How Paris Became Paris: The Invention of the Modern City