This is the lesson: Great cities are like any other living things, being born and maturing and wearying and dying in their turn.
Duh, right? Everyone who’s visited a real city feels that, one way or another. All those rural people who hate cities are afraid of something legit; cities really are different. They make a weight on the world, a tear in the fabric of reality, like … like black holes, maybe. Yeah. (I go to museums sometimes. They’re cool inside, and Neil deGrasse Tyson is hot.) As more and more people come in and deposit their strangeness and leave and get replaced by others, the tear widens. Eventually it gets so deep that it forms a pocket, connected only by the thinnest thread of … something to … something. Whatever cities are made of.
But the separation starts a process, and in that pocket the many parts of the city begin to multiply and differentiate. Its sewers extend into places where there is no need for water. Its slums grow teeth; its art centers, claws. Ordinary things within it, traffic and construction and stuff like that, start to have a rhythm like a heartbeat, if you record their sounds and play them back fast. The city … quickens.