"'Centers' are those particular identified sets, or systems, which appear within the larger whole as distinct and noticeable parts. They appear because they have noticeable distinctness, which makes them separate out from their surroundings and makes them cohere, and it is from the arrangements of these coherent parts that other coherent parts appear. The crux of the matter is this: A center is a kind of entity which can be defined only in terms of other centers. Centers are - and can only be - made of other centers." Christopher Alexander. The Nature of Order (forthcoming; Oxford University Press, 2002)