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In a general
complex system, as for example an organism or a large computer program,
certain rules of assembly are followed so that the parts cooperate and the
whole functions well. There is little formal difference between such
systems and the urban fabric. A few structural rules have evolved in the
study of complex systems. Initially stated by Herbert Simon for economics,
some were re-invented in the context of computer programming. Others
appeared independently in engineering and biology. Of the many different
possible statements of system rules, the following list is critically
relevant to urban design.
Rule 1. COUPLINGS: Strongly-coupled elements on the same scale form
a module. There should be no unconnected elements inside a module.
Rule 2. DIVERSITY: Similar elements do not couple. A critical
diversity of different elements is needed because some will catalyze
couplings between others.
Rule 3. BOUNDARIES: Different modules couple via their boundary
elements. Connections form between modules, and not between their internal
elements.
Rule 4. FORCES: Interactions are naturally strongest on the
smallest scale, and weakest on the largest scale. Reversing them generates
pathologies.
Rule 5. ORGANIZATION: Long-range forces create the large scale from
well-defined structure on the smaller scales. Alignment does not
establish, but can destroy short-range couplings.
Rule 6. HIERARCHY: A system's components assemble progressively
from small to large. This process generates linked units defined on many
distinct scales.
Rule 7. INTERDEPENDENCE: Elements and modules on different scales
do not depend on each other in a symmetric manner: a higher scale requires
all lower scales, but not vice versa.
Rule 8. DECOMPOSITION: A coherent system cannot be completely
decomposed into constituent parts. There exist many inequivalent
decompositions based on different types of units.
These eight
rules are offered as generic principles of urban form. The whole point is
to convince the reader of their inevitability in assembling a living city.
A system's development in time defines an underlying sequence. The smaller
scales need to be defined before the larger scales: their elements must
couple in a stable manner before the higher-order modules can even begin
to form and interact. Elements on the smallest scale, along with their
couplings, thus provide the foundations for the entire structure.
Requiring a hierarchy of nested scales means that not even one scale can
be missing, otherwise the whole system is unstable.
The coherence of a complex interacting system may be understood as it
connects progressively. During a short time period, strong couplings will
establish internal equilibrium in each module, with little change in the
relationship among different modules. (One analogy is the initial
formation of many small isolated crystals in a solution). Over a longer
time period, the weaker couplings between modules will take them towards a
larger-order equilibrium, while their internal equilibria are of course
maintained. The process iterates, so that on even longer time periods,
modules of modules tend towards equilibrium, and so on. The end result is
a global equilibrium state for the entire system (corresponding to a
single complex crystal).
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