Luke Wroblewski summarizes a presentation by Donna Maurer at the 7th annual Information Architecture Summit on how we use categories to conceive the world. He also links to a David Weinberger post about Maurer's talk. The material is pretty deep and the stuff of language philosophers, but the takeaway for me is how the persistent Web is changing old taxonomies.
One of the changes, as Weinberger notes in this 2004 presentation at the Library of Congress (Quicktime video), is that knowledge now often foils neat categorization, that it's all miscellaneous. How we conceive the world also depends, as Chris Anderson says, on the post-filters we apply (as opposed to expertly developed pre-filters). If you have some time, I highly recommend the Weinberger video.