ontology overrated 
- words straight from the article, which appears to have been written around 2005, entire article in link above
what we're seeing when we see the Web  is actually a radical break with previous categorization strategies,  rather than an extension of them. 
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.
yahoo:
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.  
google:
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.
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.  
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 of yours.
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.
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.
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. ... But the basic scheme gives us ways to create a globally unique  identifier for anything.  
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.  
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.  
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.
"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 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.
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.
This allows for partial, incomplete, or probabilistic merges that are  better fits to uncertain environments -- such as the real world -- than  rigid classification schemes.
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.
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.
It's all dependent on human context.  
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.
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