Over sixty years ago, Arthur Oncken Lovejoy, a professor at Johns Hopkins, gave a series of lectures that resulted in his classic work, The Great Chain of Being. Its central aim was to show that there was a:
...plan and structure of the world which, through the Middle Ages and down to the late eighteenth century...most educated men were to accept without question - the conception of the universe as a "Great Chain of Being", composed of an immense, or...infinite, number of links ranging in hierarchical order from the meagerest kind of existents...through "every possible" grade up to the ens perfectissumum.
At the top was God, of course. Then came angels and demons, then humans, then animals, then plants, minerals, and at the bottom, non-being. Within these broad categories, each and every thing had its place, depending on how much "spirit" it contained as opposed to mere "matter." Not only were rabbits ahead of fish, and gold ahead of lead, but squires were above merchants.
While the hierarchy of beings was laid out as rungs on a ladder, the theory of "correspondences" added a layer of complexity and even beauty to the notion: Different sets of rungs reflected the order of larger sets in what we would today call a fractal way: The governmental order reflected the order of the cosmos, human psychology reflected the four elements, etc.
Despite this nicely complicating wrinkle, the fundamental fact and purpose of the Great Chain of Being was to be simple and complete: Every entity had its spot in the hierarchy, every spot was filled, and there could be no movement and no vacancies ... ruling out evolution and extinction, not to mention making social mobility a crime against nature.
Why believe such a foolish thing? After all, it can't be derived from evidence. It does, however, do something that all great theories do: It unifies disparate experience. In fact, the Great Chain is precisely about showing the inner order of the diversity of entities. It unifies them not only in terms of their rank order but also in terms of their value. And it explains why there are precisely these types of creatures and not others.
Even though the Chain has gone through some serious revisions over the millennia, in one important way it has remained the same. In the 18th Century, Linnaeus re-did Aristotle's classifications, adding a couple more grand categories. But, like Aristotle, Linnaeus assumed that he was uncovering God's own way of classifying the world. Likewise, modern "cladistics" redraws Linnaeus' tree (and Stephen Jay Gould would remind us that it's more like shrubbery than a tree) according to each animal's ancestry, not according to the similarities of their anatomy, which is all Linnaeus had to go on. In all these cases, the chain or tree is assumed to represent real classifications, although the nature of the reality — God in Aristotle's or Linnaeus' eyes, Nature's in Darwin's — is different.
But now we are at a breaking point, for the digitization of knowledge makes it inescapably clear that most of the classificatory schemes that we care about are invented, not discovered. Why is this so clear? Because it's so easy to pivot the table, to switch schemes, to file ideas under multiple categories. Classifications are tools.
Further, classifications often no longer are the best guides to value. Google beats Yahoo because, while Yahoo puts everything into neatly arranged folders, Google looks at the one-to-one links that spread across the tree of knowledge like the work of a million spiders on LSD.
The overtaking of trees by webs means that instead of something getting its meaning from the bucket it's in, its meaning is determined by the billion different reasons people thought it'd be interesting to link to it. If you want to see what something is, don't look to where the Great Bucketer in the Sky put it. Instead, look to what the population of people who care about it think that it's about. That's why Google can turn up a page that doesn't even have the words on it that you're looking for: The page thought it was a maintenance manual for O-rings and didn't know that it's in fact about why the Challenger blew up. But the web of interested people knew it.
Once we recognize that classification schemes are tools and not representations of reality, they get much handier as tools. Of course, the price is giving up our place in the eternal order of the universe.