This is something everyone knows: A well-used city street is apt to be a safe street. A deserted city street is apt to be unsafe.

Cities never flourish alone. They have to be trading with other cities. My new hypothesis shows why. But also in trading with each other they can't be in too different stages of development, and they can't copy one another.

Just the way in Europe, Paris, Copenhagen, and Stockholm, and Frankfurt, possibly and Berlin, certainly, all had important roles, because of independence. Because they were depending on themselves.

Never underestimate the power of a city to regenerate.

Everybody's got a worldview, whether they know they have it or they don't. They might even get it when they are little tiny kids. Suppose they get it when they are in college, which is often the case, or in high school, whatever. Everything they learn after that or every thing they see after that, they fit it into that worldview. And they are making coherence of what's good, what's bad, what will work, what won't work, what's noble, what's ignoble, and so on... all through this filter.

Dull, inert cities, it is true, do contain the seeds of their own destruction and little else. But lively, diverse, intense cities contain the seeds of their own regeneration, with energy enough to carry over for problems and needs outside themselves.

I think that intelligent people to a great extent are captives of their time or place.