For planning:
The logic of this rigid segregation of functions is perfectly clear. It is far easier to plan an urban zone if it has just one purpose. It is far easier to plan the circulation of pedestrians if they do not have to compete with automobiles and trains. It is far easier to plan a forest if its sole purpose is to maximize the yield of furniture-grade timber. When two purposes must be served by a single facility or plan, the trade-offs become nettlesome. When several or many purposes must be considered, the variables that the planner must juggle begin to boggle the mind. Faced with such a labyrinth of possibilities, as Le Corbusier noted, “the human mind loses itself and becomes fatigued.”
Later, for organic systems:
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.” At this level one could say that Jacobs was a “functionalist,” a word whose use was banned in Le Corbusier’s studio. She asked, What function does this structure serve, and how well does it serve it? The “order” of a thing is determined by the purpose it serves, not by a purely aesthetic view of its surface order.81 Le Corbusier, by contrast, seemed to have firmly believed that the most efficient forms would always have a classical clarity and order. The physical environments Le Corbusier designed and built had, as did Brasilia, an overall harmony and simplicity of form. For the most part, however, they failed in important ways as places where people would want to live and work.
From Seeing Like a State.