In the final and most important chapter of her magnum opus, The Death and Life of Great American Cities, the great urban theorist Jane Jacobs discusses "the kind of problem a city is."
That problem, she says, is a problem of organized complexity.
A city is a complex, adaptive system, in which the decentralized actions of countless people, all influencing each other in both direct and indirect ways, result in patterns of life and activity that no policy maker could have predicted or orchestrated.
The task of growing a resilient city is thus less like math or engineering—making sure all the constituent parts (like infrastructure and public services) are in good working order, making sure problems are anticipated and addressed methodically. It's more like conservation biology, mixed with a healthy dose of art and maybe a little bit of alchemy.