Virtual spaces. One fundamental difference between organizing books within the physical space of the library and placing vectors within an abstract semantic space is that while a book can occupy only one physical location, it is trivial to maintain multiple representations of the same document. A physical object has to be in one place or another, meaning that someone (typically a cataloger at the Library of Congress) has to make a very difficult decision to determine the primary topic of each book. Think how consequential this decision is as it effects a patron’s ability to go to a semantically defined place in the stacks! This is in stark contrast to the multiple facets we can represent in electronic indices.