[Paleopsych] Clay Shirky: Ontology is Overrated -- Categories, Links, and Tags
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Tue Jun 28 01:09:10 UTC 2005
Clay Shirky: Ontology is Overrated -- Categories, Links, and Tags
Clay Shirky's Writings About the Internet
Economics & Culture, Media & Community
clay at shirky.com
Ontology is Overrated: Categories, Links, and Tags
This piece is based on two talks I gave in the spring of 2005 -- one
at the O'Reilly ETech conference in March, entitled "Ontology Is
Overrated", and one at the IMCExpo in April entitled "Folksonomies &
Tags: The rise of user-developed classification." The written version
is a heavily edited concatenation of those two talks.
Today I want to talk about categorization, and I want to convince you
that a lot of what we think we know about categorization is wrong. In
particular, I want to convince you that many of the ways we're
attempting to apply categorization to the electronic world are
actually a bad fit, because we've adopted habits of mind that are left
over from earlier strategies.
I also want to convince you that what we're seeing when we see the Web
is actually a radical break with previous categorization strategies,
rather than an extension of them. The second part of the talk is more
speculative, because it is often the case that old systems get broken
before people know what's going to take their place. (Anyone watching
the music industry can see this at work today.) That's what I think is
happening with categorization.
What I think is coming instead are much more organic ways of
organizing information than our current categorization schemes allow,
based on two units -- the link, which can point to anything, and the
tag, which is a way of attaching labels to links. The strategy of
tagging -- free-form labeling, without regard to categorical
constraints -- seems like a recipe for disaster, but as the Web has
shown us, you can extract a surprising amount of value from big messy
PART I: Classification and Its Discontents #
Q: What is Ontology? A: It Depends on What the Meaning of "Is" Is.
I need to provide some quick definitions, starting with ontology. It
is a rich irony that the word "ontology", which has to do with making
clear and explicit statements about entities in a particular domain,
has so many conflicting definitions. I'll offer two general ones.
The main thread of ontology in the philosophical sense is the study of
entities and their relations. The question ontology asks is: What
kinds of things exist or can exist in the world, and what manner of
relations can those things have to each other? Ontology is less
concerned with what is than with what is possible.
The knowledge management and AI communities have a related definition
-- they've taken the word "ontology" and applied it more directly to
their problem. The sense of ontology there is something like "an
explicit specification of a conceptualization."
The common thread between the two definitions is essence, "Is-ness."
In a particular domain, what kinds of things can we say exist in that
domain, and how can we say those things relate to each other?
The other pair of terms I need to define are categorization and
classification. These are the act of organizing a collection of
entities, whether things or concepts, into related groups. Though
there are some field-by-field distinctions, the terms are in the main
And then there's ontological classification or categorization, which
is organizing a set of entities into groups, based on their essences
and possible relations. A library catalog, for example, assumes that
for any new book, its logical place already exists within the system,
even before the book was published. That strategy of designing
categories to cover possible cases in advance is what I'm primarily
concerned with, because it is both widely used and badly overrated in
terms of its value in the digital world.
Now, anyone who deals with categorization for a living will tell you
they can never get a perfect system. In working classification
systems, success is not "Did we get the ideal arrangement?" but rather
"How close did we come, and on what measures?" The idea of a perfect
scheme is simply a Platonic ideal. However, I want to argue that even
the ontological ideal is a mistake. Even using theoretical perfection
as a measure of practical success leads to misapplication of
Now, to the problems of classification.
Cleaving Nature at the Joints #
[ The Periodic Table of the Elements ]
The periodic table of the elements is my vote for "Best.
Classification. Evar." It turns out that by organizing elements by the
number of protons in the nucleus, you get all of this fantastic value,
both descriptive and predictive value. And because what you're doing
is organizing things, the periodic table is as close to making
assertions about essence as it is physically possible to get. This is
a really powerful scheme, almost perfect. Almost.
All the way over in the right-hand column, the pink column, are noble
gases. Now noble gas is an odd category, because helium is no more a
gas than mercury is a liquid. Helium is not fundamentally a gas, it's
just a gas at most temperatures, but the people studying it at the
time didn't know that, because they weren't able to make it cold
enough to see that helium, like everything else, has different states
of matter. Lacking the right measurements, they assumed that
gaseousness was an essential aspect -- literally, part of the essence
-- of those elements.
Even in a nearly perfect categorization scheme, there are these kinds
of context errors, where people are placing something that is merely
true at room temperature, and is absolutely unrelated to essence,
right in the center of the categorization. And the category 'Noble
Gas' has stayed there from the day they added it, because we've all
just gotten used to that anomaly as a frozen accident.
If it's impossible to create a completely coherent categorization,
even when you're doing something as physically related to essence as
chemistry, imagine the problems faced by anyone who's dealing with a
domain where essence is even less obvious.
Which brings me to the subject of libraries.
Of Cards and Catalogs #
The periodic table gets my vote for the best categorization scheme
ever, but libraries have the best-known categorization schemes. The
experience of the library catalog is probably what people know best as
a high-order categorized view of the world, and those cataloging
systems contain all kinds of odd mappings between the categories and
the world they describe.
Here's the first top-level category in the Soviet library system:
A1: Classic works of Marxism-Leninism
A3: Life and work of C.Marx, F.Engels, V.I.Lenin
A5: Marxism-Leninism Philosophy
A6: Marxist-Leninist Political Economics
A7/8: Scientific Communism
Some of those categories are starting to look a little bit dated.
Or, my favorite -- this is the Dewey Decimal System's categorization
for religions of the world, which is the 200 category.
Dewey, 200: Religion
210 Natural theology
230 Christian theology
240 Christian moral & devotional theology
250 Christian orders & local church
260 Christian social theology
270 Christian church history
280 Christian sects & denominations
290 Other religions
How much is this not the categorization you want in the 21st century?
This kind of bias is rife in categorization systems. Here's the
Library of Congress' categorization of History. These are all the
top-level categories -- all of these things are presented as being
D: History (general)
DA: Great Britain
DH: Low Countries
DK: Former Soviet Union
DP: Iberian Peninsula
DR: Balkan Peninsula
I'd like to call your attention to the ones in bold: The Balkan
Peninsula. Asia. Africa.
And just, you know, to review the geography:
[ Spot the difference? ]
Yet, for all the oddity of placing the Balkan Peninsula and Asia in
the same level, this is harder to laugh off than the Dewey example,
because it's so puzzling. The Library of Congress -- no slouches in
the thinking department, founded by Thomas Jefferson -- has a staff of
people who do nothing but think about categorization all day long. So
what's being optimized here? It's not geography. It's not population.
It's not regional GDP.
What's being optimized is number of books on the shelf. That's what
the categorization scheme is categorizing. It's tempting to think that
the classification schemes that libraries have optimized for in the
past can be extended in an uncomplicated way into the digital world.
This badly underestimates, in my view, the degree to which what
libraries have historically been managing an entirely different
The musculature of the Library of Congress categorization scheme looks
like it's about concepts. It is organized into non-overlapping
categories that get more detailed at lower and lower levels -- any
concept is supposed to fit in one category and in no other categories.
But every now and again, the skeleton pokes through, and the skeleton,
the supporting structure around which the system is really built, is
designed to minimize seek time on shelves.
The essence of a book isn't the ideas it contains. The essence of a
book is "book." Thinking that library catalogs exist to organize
concepts confuses the container for the thing contained.
The categorization scheme is a response to physical constraints on
storage, and to people's inability to keep the location of more than a
few hundred things in their mind at once. Once you own more than a few
hundred books, you have to organize them somehow. (My mother, who was
a reference librarian, said she wanted to reshelve the entire
University library by color, because students would come in and say
"I'm looking for a sociology book. It's green...") But however you do
it, the frailty of human memory and the physical fact of books make
some sort of organizational scheme a requirement, and hierarchy is a
good way to manage physical objects.
The "Balkans/Asia" kind of imbalance is simply a byproduct of physical
constraints. It isn't the ideas in a book that have to be in one place
-- a book can be about several things at once. It is the book itself,
the physical fact of the bound object, that has to be one place, and
if it's one place, it can't also be in another place. And this in turn
means that a book has to be declared to be about some main thing. A
book which is equally about two things breaks the 'be in one place'
requirement, so each book needs to be declared to about one thing more
than others, regardless of its actual contents.
People have been freaking out about the virtuality of data for
decades, and you'd think we'd have internalized the obvious truth:
there is no shelf. In the digital world, there is no physical
constraint that's forcing this kind of organization on us any longer.
We can do without it, and you'd think we'd have learned that lesson by
The Parable of the Ontologist, or, "There Is No Shelf" #
A little over ten years ago, a couple of guys out of Stanford launched
a service called Yahoo that offered a list of things available on the
Web. It was the first really significant attempt to bring order to the
Web. As the Web expanded, the Yahoo list grew into a hierarchy with
categories. As the Web expanded more they realized that, to maintain
the value in the directory, they were going to have to systematize, so
they hired a professional ontologist, and they developed their
now-familiar top-level categories, which go to subcategories, each
subcategory contains links to still other subcategories, and so on.
Now we have this ontologically managed list of what's out there.
Here we are in one of Yahoo's top-level categories, Entertainment.
[ Yahoo's Entertainment Category ]
You can see what the sub-categories of Entertainment are, whether or
not there are new additions, and how many links roll up under those
sub-categories. Except, in the case of Books and Literature, that
sub-category doesn't tell you how many links roll up under it. Books
and Literature doesn't end with a number of links, but with an "@"
sign. That "@" sign is telling you that the category of Books and
Literature isn't 'really' in the category Entertainment. Yahoo is
saying "We've put this link here for your convenience, but that's only
to take you to where Books and Literature 'really' are." To which one
can only respond -- "What's real?"
Yahoo is saying "We understand better than you how the world is
organized, because we are trained professionals. So if you mistakenly
think that Books and Literature are entertainment, we'll put a little
flag up so we can set you right, but to see those links, you have to
'go' to where they 'are'." (My fingers are going to fall off from all
the air quotes.) When you go to Literature -- which is part of
Humanities, not Entertainment -- you are told, similarly, that
booksellers are not 'really' there. Because they are a commercial
service, booksellers are 'really' in Business.
[ 'Literature' on Yahoo ]
Look what's happened here. Yahoo, faced with the possibility that they
could organize things with no physical constraints, added the shelf
back. They couldn't imagine organization without the constraints of
the shelf, so they added it back. It is perfectly possible for any
number of links to be in any number of places in a hierarchy, or in
many hierarchies, or in no hierarchy at all. But Yahoo decided to
privilege one way of organizing links over all others, because they
wanted to make assertions about what is "real."
The charitable explanation for this is that they thought of this kind
of a priori organization as their job, and as something their users
would value. The uncharitable explanation is that they thought there
was business value in determining the view the user would have to
adopt to use the system. Both of those explanations may have been true
at different times and in different measures, but the effect was to
override the users' sense of where things ought to be, and to insist
on the Yahoo view instead.
File Systems and Hierarchy #
It's easy to see how the Yahoo hierarchy maps to technological
constraints as well as physical ones. The constraints in the Yahoo
directory describes both a library categorization scheme and,
obviously, a file system -- the file system is both a powerful tool
and a powerful metaphor, and we're all so used to it, it seems
[ Hierarchy ]
There's a top level, and subdirectories roll up under that.
Subdirectories contain files or further subdirectories and so on, all
the way down. Both librarians and computer scientists hit the same
next idea, which is "You know, it wouldn't hurt to add a few secondary
links in here" -- symbolic links, aliases, shortcuts, whatever you
want to call them.
[ Plus Links ]
The Library of Congress has something similar in its second-order
categorization -- "This book is mainly about the Balkans, but it's
also about art, or it's mainly about art, but it's also about the
Balkans." Most hierarchical attempts to subdivide the world use some
system like this.
Then, in the early 90s, one of the things that Berners-Lee showed us
is that you could have a lot of links. You don't have to have just a
few links, you could have a whole lot of links.
[ Plus Lots of Links ]
This is where Yahoo got off the boat. They said, "Get out of here with
that crazy talk. A URL can only appear in three places. That's the
Yahoo rule." They did that in part because they didn't want to get
spammed, since they were doing a commercial directory, so they put an
upper limit on the number of symbolic links that could go into their
view of the world. They missed the end of this progression, which is
that, if you've got enough links, you don't need the hierarchy
anymore. There is no shelf. There is no file system. The links alone
[ Just Links (There Is No Filesystem) ]
One reason Google was adopted so quickly when it came along is that
Google understood there is no shelf, and that there is no file system.
Google can decide what goes with what after hearing from the user,
rather than trying to predict in advance what it is you need to know.
Let's say I need every Web page with the word "obstreperous" and
"Minnesota" in it. You can't ask a cataloguer in advance to say "Well,
that's going to be a useful category, we should encode that in
advance." Instead, what the cataloguer is going to say is,
"Obstreperous plus Minnesota! Forget it, we're not going to optimize
for one-offs like that." Google, on the other hand, says, "Who cares?
We're not going to tell the user what to do, because the link
structure is more complex than we can read, except in response to a
Browse versus search is a radical increase in the trust we put in link
infrastructure, and in the degree of power derived from that link
structure. Browse says the people making the ontology, the people
doing the categorization, have the responsibility to organize the
world in advance. Given this requirement, the views of the catalogers
necessarily override the user's needs and the user's view of the
world. If you want something that hasn't been categorized in the way
you think about it, you're out of luck.
The search paradigm says the reverse. It says nobody gets to tell you
in advance what it is you need. Search says that, at the moment that
you are looking for it, we will do our best to service it based on
this link structure, because we believe we can build a world where we
don't need the hierarchy to coexist with the link structure.
A lot of the conversation that's going on now about categorization
starts at a second step -- "Since categorization is a good way to
organize the world, we should..." But the first step is to ask the
critical question: Is categorization a good idea? We can see, from the
Yahoo versus Google example, that there are a number of cases where
you get significant value out of not categorizing. Even Google adopted
DMOZ, the open source version of the Yahoo directory, and later they
downgraded its presence on the site, because almost no one was using
it. When people were offered search and categorization side-by-side,
fewer and fewer people were using categorization to find things.
When Does Ontological Classification Work Well? #
Ontological classification works well in some places, of course. You
need a card catalog if you are managing a physical library. You need a
hierarchy to manage a file system. So what you want to know, when
thinking about how to organize anything, is whether that kind of
classification is a good strategy.
Here is a partial list of characteristics that help make it work:
Domain to be Organized
* Small corpus
* Formal categories
* Stable entities
* Restricted entities
* Clear edges
This is all the domain-specific stuff that you would like to be true
if you're trying to classify cleanly. The periodic table of the
elements has all of these things -- there are only a hundred or so
elements; the categories are simple and derivable; protons don't
change because of political circumstances; only elements can be
classified, not molecules; there are no blended elements; and so on.
The more of those characteristics that are true, the better a fit
ontology is likely to be.
The other key question, besides the characteristics of the domain
itself, is "What are the participants like?" Here are some things
that, if true, help make ontology a workable classification strategy:
* Expert catalogers
* Authoritative source of judgment
* Coordinated users
* Expert users
DSM-IV, the 4th version of the psychiatrists' Diagnostic and
Statistical Manual, is a classic example of an classification scheme
that works because of these characteristics. DSM IV allows
psychiatrists all over the US, in theory, to make the same judgment
about a mental illness, when presented with the same list of symptoms.
There is an authoritative source for DSM-IV, the American Psychiatric
Association. The APA gets to say what symptoms add up to psychosis.
They have both expert cataloguers and expert users. The amount of
'people infrastructure' that's hidden in a working system like DSM IV
is a big part of what makes this sort of categorization work.
This 'people infrastructure' is very expensive, though. One of the
problem users have with categories is that when we do head-to-head
tests -- we describe something and then we ask users to guess how we
described it -- there's a very poor match. Users have a terrifically
hard time guessing how something they want will have been categorized
in advance, unless they have been educated about those categories in
advance as well, and the bigger the user base, the more work that user
You can also turn that list around. You can say "Here are some
characteristics where ontological classification doesn't work well":
* Large corpus
* No formal categories
* Unstable entities
* Unrestricted entities
* No clear edges
* Uncoordinated users
* Amateur users
* Naive catalogers
* No Authority
If you've got a large, ill-defined corpus, if you've got naive users,
if your cataloguers aren't expert, if there's no one to say
authoritatively what's going on, then ontology is going to be a bad
The list of factors making ontology a bad fit is, also, an almost
perfect description of the Web -- largest corpus, most naive users, no
global authority, and so on. The more you push in the direction of
scale, spread, fluidity, flexibility, the harder it becomes to handle
the expense of starting a cataloguing system and the hassle of
maintaining it, to say nothing of the amount of force you have to get
to exert over users to get them to drop their own world view in favor
The reason we know SUVs are a light truck instead of a car is that the
Government says they're a light truck. This is voodoo categorization,
where acting on the model changes the world -- when the Government
says an SUV is a truck, it is a truck, by definition. Much of the
appeal of categorization comes from this sort of voodoo, where the
people doing the categorizing believe, even if only unconciously, that
naming the world changes it. Unfortunately, most of the world is not
actually amenable to voodoo categorization.
The reason we don't know whether or not Buffy, The Vampire Slayer is
science fiction, for example, is because there's no one who can say
definitively yes or no. In environments where there's no authority and
no force that can be applied to the user, it's very difficult to
support the voodoo style of organization. Merely naming the world
creates no actual change, either in the world, or in the minds of
potential users who don't understand the system.
Mind Reading #
One of the biggest problems with categorizing things in advance is
that it forces the categorizers to take on two jobs that have
historically been quite hard: mind reading, and fortune telling. It
forces categorizers to guess what their users are thinking, and to
make predictions about the future.
The mind-reading aspect shows up in conversations about controlled
vocabularies. Whenever users are allowed to label or tag things,
someone always says "Hey, I know! Let's make a thesaurus, so that if
you tag something 'Mac' and I tag it 'Apple' and somebody else tags it
'OSX', we all end up looking at the same thing!" They point to the
signal loss from the fact that users, although they use these three
different labels, are talking about the same thing.
The assumption is that we both can and should read people's minds,
that we can understand what they meant when they used a particular
label, and, understanding that, we can start to restrict those labels,
or at least map them easily onto one another.
This looks relatively simple with the Apple/Mac/OSX example, but when
we start to expand to other groups of related words, like movies,
film, and cinema, the case for the thesaurus becomes much less clear.
I learned this from Brad Fitzgerald's design for LiveJournal, which
allows user to list their own interests. LiveJournal makes absolutely
no attempt to enforce solidarity or a thesaurus or a minimal set of
terms, no check-box, no drop-box, just free-text typing. Some people
say they're interested in movies. Some people say they're interested
in film. Some people say they're interested in cinema.
The cataloguers first reaction to that is, "Oh my god, that means you
won't be introducing the movies people to the cinema people!" To which
the obvious answer is "Good. The movie people don't want to hang out
with the cinema people." Those terms actually encode different things,
and the assertion that restricting vocabularies improves signal
assumes that that there's no signal in the difference itself, and no
value in protecting the user from too many matches.
When we get to really contested terms like queer/gay/homosexual, by
this point, all the signal loss is in the collapse, not in the
expansion. "Oh, the people talking about 'queer politics' and the
people talking about 'the homosexual agenda', they're really talking
about the same thing." Oh no they're not. If you think the movies and
cinema people were going to have a fight, wait til you get the queer
politics and homosexual agenda people in the same room.
You can't do it. You can't collapse these categorizations without some
signal loss. The problem is, because the cataloguers assume their
classification should have force on the world, they underestimate the
difficulty of understanding what users are thinking, and they
overestimate the amount to which users will agree, either with one
another or with the catalogers, about the best way to categorize. They
also underestimate the loss from erasing difference of expression, and
they overestimate loss from the lack of a thesaurus.
Fortune Telling #
The other big problem is that predicting the future turns out to be
hard, and yet any classification system meant to be stable over time
puts the categorizer in the position of fortune teller.
Alert readers will be able to spot the difference between Sentence A
and Sentence B.
A: "I love you."
B: "I will always love you."
Woe betide the person who utters Sentence B when what they mean is
Sentence A. Sentence A is a statement. Sentence B is a prediction.
But this is the ontological dilemma. Consider the following
A: "This is a book about Dresden."
B: "This is a book about Dresden,
and it goes in the category 'East Germany'."
That second sentence seems so obvious, but East Germany actually
turned out to be an unstable category. Cities are real. They are real,
physical facts. Countries are social fictions. It is much easier for a
country to disappear than for a city to disappear, so when you're
saying that the small thing is contained by the large thing, you're
actually mixing radically different kinds of entities. We pretend that
'country' refers to a physical area the same way 'city' does, but it's
not true, as we know from places like the former Yugoslavia.
There is a top-level category, you may have seen it earlier in the
Library of Congress scheme, called Former Soviet Union. The best they
were able to do was just tack "former" onto that entire zone that
they'd previously categorized as the Soviet Union. Not because that's
what they thought was true about the world, but because they don't
have the staff to reshelve all the books. That's the constraint.
Part II: The Only Group That Can Categorize Everything Is Everybody
"My God. It's full of links!" #
When we reexamine categorization without assuming the physical
constraint either of hierarchy on disk or of hierarchy in the physical
world, we get very different answers. Let's say you wanted to merge
two libraries -- mine and the Library of Congress's. (You can tell
it's the Library of Congress on the right, because they have a few
more books than I do.)
[ Two Categorized Collections of Books ]
So, how do we do this? Do I have to sit down with the Librarian of
Congress and say, "Well, in my world, Python In A Nutshell is a
reference work, and I keep all of my books on creativity together." Do
we have to hash out the difference between my categorization scheme
and theirs before the Library of Congress is able to take my books?
No, of course we don't have to do anything of the sort. They're able
to take my books in while ignoring my categories, because all my books
have ISBN numbers, International Standard Book Numbers. They're not
merging at the category level. They're merging at the globally unique
item level. My entities, my uniquely labeled books, go into Library of
Congress scheme trivially. The presence of unique labels means that
merging libraries doesn't require merging categorization schemes.
[ Merge ISBNs ]
Now imagine a world where everything can have a unique identifier.
This should be easy, since that's the world we currently live in --
the URL gives us a way to create a globally unique ID for anything we
need to point to. Sometimes the pointers are direct, as when a URL
points to the contents of a Web page. Sometimes they are indirect, as
when you use an Amazon link to point to a book. Sometimes there are
layers of indirection, as when you use a URI, a uniform resource
identifier, to name something whose location is indeterminate. But the
basic scheme gives us ways to create a globally unique identifier for
And once you can do that, anyone can label those pointers, can tag
those URLs, in ways that make them more valuable, and all without
requiring top-down organization schemes. And this -- an explosion in
free-form labeling of links, followed by all sorts of ways of grabbing
value from those labels -- is what I think is happening now.
Great Minds Don't Think Alike #
Here is del.icio.us, Joshua Shachter's social bookmarking service.
It's for people who are keeping track of their URLs for themselves,
but who are willing to share globally a view of what they're doing,
creating an aggregate view of all users' bookmarks, as well as a
personal view for each user.
[ Front Page of del.icio.us ]
As you can see here, the characteristics of a del.icio.us entry are a
link, an optional extended description, and a set of tags, which are
words or phrases users attach to a link. Each user who adds a link to
the system can give it a set of tags -- some do, some don't. Attached
to each link on the home page are the tags, the username of the person
who added it, the number of other people who have added that same
link, and the time.
Tags are simply labels for URLs, selected to help the user in later
retrieval of those URLs. Tags have the additional effect of grouping
related URLs together. There is no fixed set of categories or
officially approved choices. You can use words, acronyms, numbers,
whatever makes sense to you, without regard for anyone else's needs,
interests, or requirements.
The addition of a few simple labels hardly seems so momentous, but the
surprise here, as so often with the Web, is the surprise of
simplicity. Tags are important mainly for what they leave out. By
forgoing formal classification, tags enable a huge amount of
user-produced organizational value, at vanishingly small cost.
There's a useful comparison here between gopher and the Web, where
gopher was better organized, better mapped to existing institutional
practices, and utterly unfit to work at internet scale. The Web, by
contrast, was and is a complete mess, with only one brand of pointer,
the URL, and no mechanism for global organization or resources. The
Web is mainly notable for two things -- the way it ignored most of the
theories of hypertext and rich metadata, and how much better it works
than any of the proposed alternatives. (The Yahoo/Google strategies I
mentioned earlier also split along those lines.)
With those changes afoot, here are some of the things that I think are
coming, as advantages of tagging systems:
* Market Logic - As we get used to the lack of physical constraints,
as we internalize the fact that there is no shelf and there is no
disk, we're moving towards market logic, where you deal with
individual motivation, but group value.
As Schachter says of del.icio.us, "Each individual categorization
scheme is worth less than a professional categorization scheme.
But there are many, many more of them." If you find a way to make
it valuable to individuals to tag their stuff, you'll generate a
lot more data about any given object than if you pay a
professional to tag it once and only once. And if you can find any
way to create value from combining myriad amateur classifications
over time, they will come to be more valuable than professional
categorization schemes, particularly with regards to robustness
and cost of creation.
The other essential value of market logic is that individual
differences don't have to be homogenized. Look for the word
'queer' in almost any top-level categorization. You will not find
it, even though, as an organizing principle for a large group of
people, that word matters enormously. Users don't get to
participate those kind of discussions around traditional
categorization schemes, but with tagging, anyone is free to use
the words he or she thinks are appropriate, without having to
agree with anyone else about how something "should" be tagged.
Market logic allows many distinct points of view to co-exist,
because it allows individuals to preserve their point of view,
even in the face of general disagreement.
* User and Time are Core Attributes - This is absolutely essential.
The attitude of the Yahoo ontologist and her staff was -- "We are
Yahoo We do not have biases. This is just how the world is. The
world is organized into a dozen categories." You don't know who
those people were, where they came from, what their background
was, what their political biases might be.
Here, because you can derive 'this is who this link is was tagged
by' and 'this is when it was tagged, you can start to do inclusion
and exclusion around people and time, not just tags. You can start
to do grouping. You can start to do decay. "Roll up tags from just
this group of users, I'd like to see what they are talking about"
or "Give me all tags with this signature, but anything that's more
than a week old or a year old."
This is group tagging -- not the entire population, and not just
me. It's like Unix permissions -- right now we've got tags for
user and world, and this is the base on which we will be inventing
group tags. We're going to start to be able to subset our
categorization schemes. Instead of having massive categorizations
and then specialty categorization, we're going to have a spectrum
between them, based on the size and make-up of various tagging
* Signal Loss from Expression - The signal loss in traditional
categorization schemes comes from compressing things into a
restricted number of categories. With tagging, when there is
signal loss, it comes from people not having any commonality in
talking about things. The loss is from the multiplicity of points
of view, rather than from compression around a single point of
view. But in a world where enough points of view are likely to
provide some commonality, the aggregate signal loss falls with
scale in tagging systems, while it grows with scale in systems
with single points of view.
The solution to this sort of signal loss is growth. Well-managed,
well-groomed organizational schemes get worse with scale, both
because the costs of supporting such schemes at large volumes are
prohibitive, and, as I noted earlier, scaling over time is also a
serious problem. Tagging, by contrast, gets better with scale.
With a multiplicity of points of view the question isn't "Is
everyone tagging any given link 'correctly'", but rather "Is
anyone tagging it the way I do?" As long as at least one other
person tags something they way you would, you'll find it -- using
a thesaurus to force everyone's tags into tighter synchrony would
actually worsen the noise you'll get with your signal. If there is
no shelf, then even imagining that there is one right way to
organize things is an error.
* The Filtering is Done Post Hoc - There's an analogy here with
every journalist who has ever looked at the Web and said "Well, it
needs an editor." The Web has an editor, it's everybody. In a
world where publishing is expensive, the act of publishing is also
a statement of quality -- the filter comes before the publication.
In a world where publishing is cheap, putting something out there
says nothing about its quality. It's what happens after it gets
published that matters. If people don't point to it, other people
won't read it. But the idea that the filtering is after the
publishing is incredibly foreign to journalists.
Similarly, the idea that the categorization is done after things
are tagged is incredibly foreign to cataloguers. Much of the
expense of existing catalogue systems is in trying to prevent
one-off categories. With tagging, what you say is "As long as a
lot of people are tagging any given link, the rare tags can be
used or ignored, as the user likes. We won't even have to expend
the cost to prevent people from using them. We'll just help other
users ignore them if they want to."
Again, scale comes to the rescue of the system in a way that would
simply break traditional cataloging schemes. The existence of an
odd or unusual tag is a problem if it's the only way a given link
has been tagged, or if there is no way for a user to avoid that
tag. Once a link has been tagged more than once, though, users can
view or ignore the odd tags as it suits them, and the decision
about which tags to use comes after the links have been tagged,
* Merged from URLs, Not Categories - You don't merge tagging schemes
at the category level and then see what the contents are. As with
the 'merging ISBNs' idea, you merge individual contents, because
we now have URLs as unique handles. You merge from the URLs, and
then try and derive something about the categorization from there.
This allows for partial, incomplete, or probabilistic merges that
are better fits to uncertain environments -- such as the real
world -- than rigid classification schemes.
* Merges are Probabilistic, not Binary - Merges create partial
overlap between tags, rather than defining tags as synonyms.
Instead of saying that any given tag "is" or "is not" the same as
another tag, del.icio.us is able to recommend related tags by
saying "A lot of people who tagged this 'Mac' also tagged it
'OSX'." We move from a binary choice between saying two tags are
the same or different to the Venn diagram option of "kind of
is/somewhat is/sort of is/overlaps to this degree". That is a
really profound change.
Tag Distributions on del.icio.us #
Here's something showing what I mean about the breakdown of binary
[ Tags per user ]
This is a chart based on a small sample of links from the del.icio.us
front page, taken during a 2-hour window. The X axis is the 64 users
who posted links during that period. The Y axis is the total number of
discrete kinds of tags that those users have ever used in their
history on del.icio.us.
The chart shows a great variability in tagging strategies among the
various users. The user all the way to the left has an enormous number
of unique tags, almost 600 of them. Then there's this group of people
who are not quite power taggers but who tag quite a bit, and of course
to the right of them there's the characteristic long tail of people
who use many fewer tags than the power taggers. (Because this is a
two-hour snapshot, it has a natural bias towards frequent del.icio.us
users. I'm trying to get a larger data set. My guess is the tail goes
out quite a bit further than this.) But this is what organization
looks like when you turn it over to the users -- many different
strategies, each of which works in its own context, but which can also
[ A single user's tags ]
This is a single user's tags. From here, you can tell something about
this person -- he or she is obviously a Flash programmer -- the
commonest tag here is Flash, followed by a number of other frequently
used tags mainly related to programming. Like the front page, this
distribution has the organic signature. Experts don't catalog this
way; experts who learn how to catalogue produce much more consistent
labeling. Here, it's whatever the user thought would help them
remember the link later.
You can see there's a tag "to_read". A professional cataloguer would
look at this tag in horror -- "This is context-dependent and
temporary." Well, so was the category "East Germany." Once you expand
your time scale to include the actual life of the categorization
scheme itself, you recognize that the distinction between temporary
and permanent is awfully vague. There isn't in fact a binary condition
of a tag that can or cannot survive any kind of long-term examination.
[ Different tag 'signatures' for different URLs ]
Then there's this set of graphs. This is to me in a way the most
interesting and least well understood part of the del.icio.us right
now -- these are two different URLs and the tags that a whole group of
users applied to them. The graph at the bottom left refers to a site
for downloading old versions of programs that are no longer supported.
You can see here that there is broad communal consensus. 140 people
tagged this Software. Then, the next commonest tag, with only 20
occurrences, is Windows, then Old, then Download, and so forth. For
this URL, there's a core consensus -- this link is about software --
and after that one bit of commonality, there is a really sharp, clear
fall off in tags.
The graph at the upper right, by contrast, shows the tags for a page
detailing how to embed standing searches in Gmail. You can see the
much smearier distribution, with a much less sharp fall-off. The
consensus view is that this link is about more kinds of things than
the software download link is, or, rather, occupies more contexts for
del.icio.us users than the software download link does.
Looking at this sort of data, we can start to say, of particular URLs,
that the users tagging this URL either did or did not center around a
certain core tags, with this degree of certainty, and, thanks to the
time stamps, we can even start to understand how the distribution of a
URLs tags changes over time. It was 5 years between the spread of the
link and Google's figuring out how to use whole collections of links
to create additional value. We're early in the use of tags, so we
don't yet have large, long-lived data sets to look at, but they are
being built up quickly, and we're just figuring out how to extract
novel value from whole collections of tags.
Organization Goes Organic #
We are moving away from binary categorization -- books either are or
are not entertainment -- and into this probabilistic world, where N%
of users think books are entertainment. It may well be that within
Yahoo, there was a big debate about whether or not books are
entertainment. But they either had no way of reflecting that debate or
they decided not to expose it to the users. What instead happened was
it became an all-or-nothing categorization, "This is entertainment,
this is not entertainment." We're moving away from that sort of
absolute declaration, and towards being able to roll up this kind of
value by observing how people handle it in practice.
It comes down ultimately to a question of philosophy. Does the world
make sense or do we make sense of the world? If you believe the world
makes sense, then anyone who tries to make sense of the world
differently than you is presenting you with a situation that needs to
be reconciled formally, because if you get it wrong, you're getting it
wrong about the real world.
If, on the other hand, you believe that we make sense of the world, if
we are, from a bunch of different points of view, applying some kind
of sense to the world, then you don't privilege one top level of
sense-making over the other. What you do instead is you try to find
ways that the individual sense-making can roll up to something which
is of value in aggregate, but you do it without an ontological goal.
You do it without a goal of explicitly getting to or even closely
matching some theoretically perfect view of the world.
Critically, the semantics here are in the users, not in the system.
This is not a way to get computers to understand things. When
del.icio.us is recommending tags to me, the system is not saying, "I
know that OSX is an operating system. Therefore, I can use predicate
logic to come up with recommendations -- users run software, software
runs on operating systems, OSX is a type of operating system -- and
then say 'Here Mr. User, you may like these links.'"
What it's doing instead is a lot simpler: "A lot of users tagging
things foobar are also tagging them frobnitz. I'll tell the user
foobar and frobnitz are related." It's up to the user to decide
whether or not that recommendation is useful -- del.icio.us has no
idea what the tags mean. The tag overlap is in the system, but the tag
semantics are in the users. This is not a way to inject linguistic
meaning into the machine.
It's all dependent on human context. This is what we're starting to
see with del.icio.us, with Flickr, with systems that are allowing for
and aggregating tags. The signal benefit of these systems is that they
don't recreate the structured, hierarchical categorization so often
forced onto us by our physical systems. Instead, we're dealing with a
significant break -- by letting users tag URLs and then aggregating
those tags, we're going to be able to build alternate organizational
systems, systems that, like the Web itself, do a better job of letting
individuals create value for one another, often without realizing it.
Thank you very much.
Thanks to Alicia Cervini for invaluable editorial help.
Clay Shirky's Writings About the Internet
Economics & Culture, Media & Community, Open Source
clay at shirky.com
2. mailto:clay at shirky.com
18. mailto:clay at shirky.com
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