"A fundamental mistake that urban planners made, Jacobs claims, was to infer functional order from the duplication and regimentation of building forms: that is, from purely visual order. Most complex systems, on the contrary, do not display a surface regularity; their order must be sought at a deeper level. "To see complex systems of functional order as order, and not as chaos, takes understanding. The leaves dropping from the trees in the autumn, the interior of an airplane engine, the entrails of a rabbit, the city desk of a newspaper, all appear to be chaos if they are seen without comprehension. Once they are seen as systems of order, they actually look different."