The following sections are excerpted with permission from Chapter 1 of Toby Hemenway’s new book The Permaculture City, published by Chelsea Green.

When a permaculturist sees words such as “function” and “synergy,” it sets off lightbulbs in his or her head. Function, for example, indicates a relationship, a connection between two or more elements. A road functions to move traffic, thus the road has a relationship with vehicles, and it mediates the movement—that is, it makes connections—between the traffic, its origin, and its destination. Knowing a function, in turn, leads us to identify the items and processes necessary to fill that function and also points to the yields created when that function is filled. Thinking in terms of functions, then, is a powerful leverage point, because it identifies needs, yields, relationships, and goals, and it helps us spot blockages, missing elements, buildup of waste, and inefficiencies in the various flows and linkages that are part of that function’s workings.
 
This means that when we look at cities, their residents, and the other components of urban life in terms of their functions, we can spot the factors that influence how well they are able to perform those functions. Then we can study, understand, and direct those factors and influences in ways that will create and enhance the functions and properties of cities that are beneficial, such as community-building public plazas, parks, and structures; open and supportive marketplaces; and habitat-creating green space; as well as human elements such as responsive policy processes. We can also spot and damp down the negative factors. Once we’ve done this, the next step is to evaluate, to see how well our changes have moved us toward a more livable, and life-filled, environment. That is the heart of design.
 
The importance of the three primary functions of cities—inspirational gathering space, security, and trade—is also visible in the negative. When cities grow ugly or inhumanly scaled, when they are crime-ridden or prone to raids, or when their industries fail, urbanites retreat if they can to the suburbs, the hinterlands, or another more functional city. Those who can’t leave often crowd—or are forced—into ghettos and enclaves. The movement of people in and out of a city is useful feedback about how well that city functions and what needs to be redesigned…
 
 
Cities as Complex Systems
 
The sciences of complexity studies arose in the 1960s and 1970s and spread, because they were so widely applicable, from the arid realms of theoretical physics and mathematics to other disciplines. A subdiscipline of urban planning, sometimes called complexity theory of cities, emerged in the 1980s and has since generated a blizzard of publications and experiments in urban design. I will give an overview of the origins and tenets of complexity theory of cities as it relates to permaculture. For those interested in exploring the intersection of urban design with complexity theory in more detail than I can offer here, a good place to start is an anthology of articles collected under the title Complexity Theories of Cities Have Come of Age, edited by Juval Portugali and others.
 
Understanding that cities are a form of complex adaptive system has helped urbanists restore some vibrancy to moribund metropolises, so it’s worth understanding a little about these systems. The general “messiness” of cities has been irritating urban theorists and planners for centuries, but it wasn’t until recently that urbanists truly understood that it is just that messiness that gives cities their life.
 
The urge to rationalize and give order to cities—which, incidentally, culminated in the dehumanizing urban-renewal projects of the 1960s—has its seeds back in the Enlightenment era. Philosophers and scientists of that day, inspired by the successes of Newton, Galileo, and Kepler at finding simple laws that explained and predicted mechanical action, began thinking of nature and the universe as a machine that could be dissected, rebuilt, and controlled. Once they saw that planets and falling bodies operated by simple rules, some of them began extending the machine metaphor to the living world.  Soon farming and forestry were remade in the image of the machine, and this mechanical worldview spread to human systems as well. The standardized, abstract measurements of the metric system supplanted local and traditional units that once kept their uses connected to natural objects and activities. An acre, for example, was the area of flat land that a pair of oxen could plow in a day; an inch was the length of three grains of barley laid end to end. A meter is just, well, a meter—and since the 1983 General Conference on Weights and Measures, defined as, “the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.” How’s that for abstract?
 
Tested land-use customs that had been culture- and site-specific were swept aside by nationwide property laws, official languages taught in state schools extinguished dialects and indigenous speech, and major cities such as Paris and Washington, DC, were rebuilt on rigid geometric patterns.
 
This attempt to impose a clockwork order on the confusing welter of urban life, while making cities more comprehensible to travelers and tax officials, reached its peak in the neighborhood-razing visions of New York’s Robert Moses, the sterile facades and inhuman whole-city plans of Le Corbusier, and the crime-ridden high-rise projects of south Chicago and countless other cities. As the failures of what has been called high modernism became obvious in the 1970s and 1980s, architects, planners, officials, and urban dwellers began to see that a machine city is a dead city.
 
Right at that time, though, several countering forces were emerging. One was an activist revolt against large-scale urban planning. As so often happens in the simultaneous emergence of parallel ideas whose time has come, this grassroots movement was also gaining academic legitimacy in work by theorists in the developing new complexity sciences. Mathematicians, ecologists, economists, and planners alike began to spot the consonance between complex systems such as weather, forests, neural networks, markets, and cities. Some of these complex systems could adapt and learn, while others, like the weather, could not. The former came to be called complex adaptive systems, or CAS. Researchers soon determined that to be able to learn, adapt, and evolve, CAS needed to possess certain features:
 
1.  They are composed of autonomous agents; that is, their parts work according to their own internal operating rules, whether they are nerve cells, trees, or people.
 
2.  These agents interact with each other according to certain (often simple) rules. A rule for a bird in a flock may be, “Keep the bird ahead of you at a 45-degree angle and 3 feet away.” These simple rules can result in stunningly complex behaviors, as anyone can attest who has watched a shimmering flock of birds spin patterns against the sky.
 
3.  Those new behaviors are an example of emergence, which is the appearance of novel properties that can’t be predicted by studying the parts in isolation. Watching a single bird in flight would never let you predict the intricate, captivating dance of a swooping flock of birds. Studying one cell of a slime mold would never suggest that as a group they can merge to fashion a bizarre mushroomlike colonial structure for reproduction.
 
4.  The agents respond to changes in their environment via feedback. They sense some of the effects of their actions, which allows them to adapt and learn.
 
5.  CAS usually exhibit homeostasis; that is, they self-regulate and “tune” their behavior to certain states that are preferred over other, less stable states, and they can return to these states after a disturbance. These states are usually far from equilibrium. A mammal, for example, maintains its body temperature independent of both the air temperature and how hard it is exercising. If it were at equilibrium, it would be at air temperature—and it would be dead.
 
6.  These systems maintain themselves in a rich, possibility-filled region between perfect order and total randomness that complexity thinkers call the edge of chaos. An organism, for example, contains proteins that are made to a specific pattern but are constantly moving in and out of that pattern as they are built up and broken down in metabolism. But metabolism isn’t chaotic. It follows specific pathways and rules. We can see this also in our genes. They generally are built to a set DNA sequence and pattern, but occasional mutation and regular recombination permit new possibilities to emerge. Perfect order is dead, while complete chaos allows no structure. Life and other complex adaptive systems attune themselves to the fecund, creative place between frozen order and seething randomness, to the edge of chaos, and thrive there. Healthy cities do the same.
 
In summary, CAS contain many autonomous parts, they respond to changes via feedback, and they form self-organizing, self-maintaining assemblages that display emergent properties. So how do the principles of CAS apply to urban permaculture?
 
Those principles suggest that rigid planning that leaves no room, or even not enough room, for spontaneous self-organization will create sterile cities. Strict top-down planning is anathema to CAS, including cities; it imposes a rigidity that eliminates adaptability and spontaneity. On the other hand, pure bottom-up accretion of elements with no rules or pattern at all approaches chaos and can result in grossly unequal distribution of resources, incoherent layout, gentrification, food deserts, and the other ills that plague many cities. Thus urban design methods that provide enough organization in the form of simple rules but create the conditions for spontaneity to occur can take advantage of the ways that cities behave as CAS. What does that look like?
 
One of the first to grasp the importance of urban life’s lack of tidiness was Patrick Geddes, a biologist who later turned to sociology and urban planning. Geddes was a student of Thomas Huxley, the man known as “Darwin’s bulldog” for his fierce defense of the theory of natural selection, and Geddes brought his own appreciation for evolution and life’s spontaneity to urban design. During the late nineteenth century, when Geddes was practicing, the common view was that cities were simply “architecture writ large,” mechanical elements assembled on a large scale. Geddes taught that every city evolves in both a historical context and a unique geographical setting, and any planning that ignores or attempts to remake these will harm those who live there. But Geddes was nearly a lone voice against the rising influence of those who saw the city as a machine, and their views dominated the first six decades of the twentieth century.
 
Figure 1-1. Emergence in action. The slime mold Dictyostelium germinates from spores as individual cells that remain independent until food becomes scarce. At that point the cells aggregate and can move as a multicellular organism in the pseudoplasmodium or slug stage. This “slug” slithers to a well-lighted, open place and transforms into a mushroomlike fruiting body that then releases spores. The slug and the collective fruiting body possess properties not present in the individual cells, such as the ability to form complex shapes, solve mazes (in the slug phase), and release spores (the fruiting body). Illustration by Elara Tanguy