[greenyoda]

> Is there a physical correlate for "tags"? If digital files can have multiple tags, then it seems not; in the physical world, the same exact file simply couldn't be in multiple folders at once.

This concept is much older than digital files. For example, in a traditional card catalog that you'd find in a library, the same book could be indexed under many different subject headings (each of which would correspond to a "tag").

For example, I might find a biography of Marie Curie by looking in the card catalog under "biography", "physics", "chemistry", "Nobel Prize winners" or "Marie Curie". It would also be indexed under the book's title and any of the authors' names.