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April 26, 2004

A City Is Not A Tree

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Posted by Clay Shirky

It’s a moment of disorientation I’ve had a couple of times — you find a great piece of writing, and think “Wow, this is really going to change things!”, only to discover that it is in fact decades old. The clash of historical vertigo with Internet Now is both wonderful and daunting.

I had that moment yesterday with Christopher Alexander’s A City Is Not A Tree from 1965. Alexander argues that the hallmark of designed cities (Mesa City, Brasilia) is that their builders invariably gravitate to tree-structures, where all sub-units of a similar type roll-up into a single super-unit, und so weiter, which creates an artificial and ultimately damaging simplification. He contrasts this with the structure of organic cites (London, NYC), which are organized as semi-lattices, where overlap and shared function is the order of the day.

Whenever we have a tree structure, it means that within this structure no piece of any unit is ever connected to other units, except through the medium of that unit as a whole.

The enormity of this restriction is difficult to grasp. It is a little as though the members of a family were not free to make friends outside the family, except when the family as a whole made a friendship.

In simplicity of structure the tree is comparable to the compulsive desire for neatness and order that insists the candlesticks on a mantelpiece be perfectly straight and perfectly symmetrical about the centre. The semilattice, by comparison, is the structure of a complex fabric; it is the structure of living things, of great paintings and symphonies.

It must be emphasized, lest the orderly mind shrink in horror from anything that is not clearly articulated and categorized in tree form, that the idea of overlap, ambiguity, multiplicity of aspect and the semilattice are not less orderly than the rigid tree, but more so. They represent a thicker, tougher, more subtle and more complex view of structure.
Like the 1970 Jo Freeman essay on group structure I pointed to as my inaugural post, A City Is Not A Tree is resonant in part because Alexander is describing the world we live in without having seen it.

I have an intuition that this essay says something important about planned vs grown communities in general, even when they meet outside the boundaries of real space and even when the architecture in question is an architecture of machines, but I won’t try to pin that down here —- the material needs at least a re-reading before trying to work with the ideas.

Go. Hit print.

Comments (9) + TrackBacks (0) | Category: social software


1. AJ Kim on April 26, 2004 6:33 PM writes...

Thanks for shining light on this wonderful piece, Clay. I share your intuition about planned (top-down) vs grown (bottom-up) communities - and I think great community-building is actually a balance between the two, not an either/or.

For me, the differentiation between tree-structures & lattice-structures is a big 'AHA' because it's about data-structures and linking rules for growing community infrastructure.

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2. Roger Eaton on April 26, 2004 8:19 PM writes...

Clay, I have been working with a notion that I call "bottom up hierarchy". It has the hierarchical structure needed for effectiveness and is easy to construct, yet is organic with the properties of the semilattice and is very much consonant with democratic principles.

The application is a peer-to-peer system, still in design, with built in item ratings and an emphasis on messaging (as opposed to, say music swapping, tho that would be possible, too).

Network connections are to be made manually by agreement between node owners. Between any pair of connected nodes, items can move in either direction, but item-ratings must always flow in only one direction. This restriction of ratings flow to a single direction is the simple rule that creates the hierarchy.

That is the only rule. Any node may connect to any number of other nodes and there is no rule that a node must send ratings as well as receive them, so although a hierarchy must result, it may and likely will end up being a hierarchy with multiple apexes. Curiously, a separatist node that does not want to connect up to the global top, can still have its own sub-nodes connect up, so it can maintain a connection with the whole while preserving a form of autonomy.

An example shows the semilattice effect. Say that we have a global hierarchy of women's groups, connecting up to a global voice of womankind. At the same time we have a global hierarchy of geographical regions. As needed, the women's groups in a particular geographical region can band together to form a women's node for that geographical region. Or, a women's node in a particular geographical region can form sub-nodes by political party, so that a Green sub-node would connect to the Green Party, and the Republican sub-node would connect to the Republican party for the region and so forth.

I think I am onto something simple and powerful here and would love to see some discussion.

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3. Francis Hwang on April 27, 2004 1:38 AM writes...

I was exposed to Christopher Alexander through the scrim of eastern-mysticism-cum-object-oriented-design-patterns at the original wiki, and it surprised how little he's read in the world of social software. If you really want to know about bottom-up design you should get your hands on his book The Timeless Way of Building. In it he describes the process of designing a 25,000 square foot psychiatric complex in California, which he did without top-down designs of almost any kind. He simply took some of the staff out to the ground for a week, and kept asking them questions like "When people are driving to the clinic where should they turn to enter the grounds?" and "What part of the complex should function as the main building?" and recorded the results of their discussions. The Timeless Way of Building was published in 1979, and reading it you wonder if somewhere along the line a war was fought in architecture and city planning, and that the wrong side won ...

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4. Roger Eaton on April 27, 2004 2:24 AM writes...

Well, this is interesting -- looking up Alexander in Amazon, here is a quote from his A Pattern Language: Towns, Buildings, Construction: "We begin with that part of the language which defines a town or community. These patterns can never be "designed" or "built" in one fell swoop--but patient piecemeal growth, designed in such a way that every individual act is always helping to create or generate these larger global patterns, will, slowly and surely, over the years, make a community that has these global patterns in it. *** The first ninety-four patterns..."

Wow -- 94 patterns, and these just the ones dealing with "large scale structures of the environment".

What would be a good set of patterns for a global online community?

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5. Seb on April 27, 2004 10:28 AM writes...

Since we're talking about patterns and social software:

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6. Francis Hwang on April 28, 2004 10:35 AM writes...

And don't forget the Meatball wiki:

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7. Seb on April 29, 2004 9:30 AM writes...

And don't miss Phil Jones:

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8. phil jones on May 3, 2004 1:51 PM writes...

A must-read companion to "A City is not a Tree" is Phil Agre's 'Hierarchy and History in Simon's "Architecture of Complexity" '(

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9. Stefan Jones on May 5, 2004 7:54 PM writes...

So . . . A Tree Doesn't Grow Like Brooklyn?

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