Each time we move to a new city, we make memories as the city slowly takes shape in our minds. Every new place we locate — the closest grocery store, the post office, rendezvous points with friends — is a new point on the map. Wayfinding a new city is an experience you can never get back. Once you’re familiar with the space or place, it’s gone.

Since moving out on my own, I’ve gravitated toward cities: Seattle, Portland, San Francisco, San Diego, Austin, Atlanta, Chicago. Externalized memories built in brick and concrete. It makes me think of a passage from Steve Erickson’s novel Days Between Stations:

“What is the importance of placing a memory? he said. Why spend that much time trying to find the exact geographic and temporal latitudes and longitudes of the things we remember, when what’s urgent about a memory is its essence?”

Our cities, those densely populated spaces of our built environment, have always been slowly redefining themselves. In 1981, there were the nine nations of North America. In 1991, the Edge Cities emerged. In 2001, we witnessed the worst intentions of a tightly networked community that lacked physical borders—what Richard Norton calls a “feral city,” a community that’s beyond the reach of law and order.

Our capital-driven, networked societies produce, more than anything else, ephemeral things — that is, things that are built, not to last, but to disappear and be displaced by newer versions of themselves. As David Byrne wrote in his Bicycle Diaries, cities “are physical manifestations of our deepest beliefs and our often unconscious thoughts, not so much as individuals, but as the social animals we are.” If our collective consciousness is flitting and flickering from one thing to the next, so shall our cities follow suit.

From flash mobs to terrorist cells, communities can now quickly toggle between virtual and physical organization. How long before our cities do the same?