So here is what’s radical about what Shirky protends: if I want to, I can tag my bookmarks with any vocabulary I chose. So here goes, a sample of my tags: “hdfjkfb”, “orjfkido”, “hjfoå”, “krlofpke”. So this certainly ensures that no one else knows my system, and if everyone else does the same, I won’t know theirs. But how useful would this be to anyone?? So at this rather radical extreme the Shirky claim is without content.
But maybe the example is too radical because the notion of vocabulary precludes the use of random tags. Fine. Let us think of a different example which is perhaps closer to the spirit of Clay’s idea. So I make up a system that only I know, where all interesting sites end with “xyz”, technical sites begin with “krp”, and so on. Everyone else can make up their own system, and no one knows anybody else’s. But an immediate problem with this is that it is cheating … it smuggles in the more natural English vocabulary via the back door by simply equating each English term with an expression in the new “vocabulary system”. Still, this is private knowledge so maybe that is O.K. for the example. But this leads to the interesting hypothesis: suppose you made up vocabularies like “krp…”, “…xyz”, and so on, and got users to adopt them. These can be mapped onto their “parent” vocabularies that they are derived from (remember “krp…” = “technical site”, etc.). To the individual user, each system would be equivalent (more or less, as Katie Melua would say). But which vocabularies would make a better del.icio.us?

I think it is pretty clear that we DO need to know SOMETHING about everyone else’s bookmark system! The success of a “social bookmarking system” depends on the fact that we do understand each others vocabularies (to some extent), and can extract value from that shared understanding.

What I have been arguing is that the aggregate data shows us something about what we know about everyone else’s bookmark system. Why are some terms so “natural”? Do all “natural” terms become popular? Are all “natural” terms natural in the same way, or are there different categories according to which terms can naturally relate to their categories (a bit like facets in taxonomies).
So my position is that folksonomies can provide interesting data which can give a clue about the way humans organize knowledge, AND about the ways in which we share each other’s organizational systems. My Ph.D. was originally in Cognitive Psychology, and during my studies I came to believe in the hypothesis that mental architecture fundamentally shapes our perceptions and organization of the world in which we live. Further, essential aspects of the mental architecture are fixed and therefore shared by all humans, which is what makes communication and shared understanding possible. The overlap is not perfect. I say Library of Congress, but Clay wants to say LOC. Good for him. But pity the poor soul who calls it “the square root of negative 2”!
The mind creates categories, because that is what minds do (there is an awful lot more to say about that!). The mental architecture enforces the range of possible ontologies and taxonomies that we can bring to bear on the understanding of our universe. All humans share fundamental aspects of mental architecture and therefore properties of possible taxonomies. Folksonomies provide a fantastic window into the worshop of the mental taxonomist!