"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."

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In trading with each other cities can't be in too different stages of development, and they can't copy one another. Backward cities, or younger cities, or newly forming cities in supply regions, have to develop to a great extent on one another's shoulders. This is one of the terrible things about empires. Empires want them only to trade with the empire, which doesn't help them at all. It's just a way of exploiting them.

— Jane Jacobs