Culture is born of the exchange of ideas. The greatest cities of the past were those which acted as meeting places for diverse cultures and allowed them to flourish: in the days of predominantly overland travel that meant cities such as Rome, Constantinople and Xi’an. To take a simple example, great food is never invented in isolation. As the historians of Italian food Alberto Capatti and Massimo Montanari have observed, identity derives from difference in the sense of relations with others. Thus in food terms “local” identity comes from the meeting of cultures and products being exchanged. An exclusive local product has no geographical identity since it derives from de-localisation; to use their example, the "mortadella di Bologna" is thus defined only when it is is sold beyond the area in which it is produced.
The gastronomy of each of those cities bears traces of that of the other three, in terms of raw product, spices and condiments, and preparation methods.