Navigability is a metaphor used to describe the user side of how systems attempt to facilitate users' logical access to library content. For example, many libraries organize content through classification, indexing, etc. via the Library of Congress Classification scheme. The problem facing librarians when this classification scheme was developed (as a solution to that problem at that time) involved user access to the physical location of the content as well as logical location.

Even now, from a normal user's or potential user's perspective, LC is an abstract and arbitrary way of relating one unit of content to another (e.g., why is sociology necessarily next to communication?). LC, of course, facilitated some browsing conditions where a user could look in the physical region for helpful stuff but we have no such intuitive capability in digital systems. The situation we must address has changed, radically.

• From a user perspective, digital libraries are how users find content they feel will be useful to them in addressing an interest, solving a problem, etc. Period.

• From a technical perspective, physical location is no longer a user concern. Although WE need to be able to find a particular "piece" of content once a user has asked for it, there is no reason for the user to understand how the system works.

• From a practical perspective, library automation has all but removed the human being (i.e., reference librarian) from the users' search process. The "translation" of a human need into system-specific, matchable terms by the human intermediary has been removed from current systems.

• From a linguistic perspective where the concern is the exchange of meaning (e.g., a user specifying his/her information need and projected use to the system which is then "matched" to the content organization scheme), all current content organization schemes are incomplete. In order for meaning to be exchanged, two components are necessary: topic, or the aboutness of the meaning; and comment, or how that topic fits into the user's situation, problem, etc. We currently use only topic (i.e., subject, author, title, publisher, date, etc. that point to "what" the content is) to organize content. Boolean logic and other techniques intended to improve the specificity of a user need are operating on topic (in the linguistic sense) alone. If topic "points to" what the user is interested in and comment indicates the "fit" with the user situation, there are many meanings that cannot be expressed simply by pointing - we need more complex, dynamic gestures.

Systems fail because they are not used. Period. Systems are not used because what users need is not there or else it is too difficult for them to find. The immense "size" of digital collections and their interconnectivity, exacerbate this problem because even the systems no longer know what is "in" the collection. We know from the last ten years or so of the diffusion of personal computing across society that users will simply not take the time to learn what they perceive as arbitrary technical aspects of access to their use of systems. So, the most logical direction for us to pursue in facilitating usability design and subsequent evaluation in digital libraries is to address the way the content is organized and how that organization is represented to the users. The higher-order concept I am suggesting for a user-centered approach to our design and evaluation efforts is the ease of the users' navigation as perceived by users (i.e., users know "where" they are and believe they have strategies for movement that will be successful).

The above discussion provides us with more specific design criteria for navigation . For example, the user representation should ideally "match" what is in the users heads so they don't have to learn anything arbitrary or complicated (because they won't anyway) in order for the system to be usable to them.

To the extent that there are system complexities in retrieving a cluster of bits for the users, we need to keep that complexity from interfering with the users' navigation activities.

The navigation metaphor implies a content organization "space" through which users will move. "Space" implies that adjacency has some meaning as it does in physical space. However, unlike LC, the "dimensions" that make up this space must themselves be meaningful to all users on an a priori basis. I believe that the issue of the nature of this space is the single most important and, unfortunately, most difficult, issue facing the coherent design (and subsequent evaluation) of digital information and communication systems. I do have some ideas and some research that begins to address the problem of developing content organization "dimensions" which can be represented to users as a coherent space that should require little, if any, training for users to be able to navigate through it.

The basis of my view originates with Dervin's metaphor of cognitive movement to represent how users perceive the world; a Prague functional linguistic model of meaning and the exchange of meaning; Kim's view of co-oriented group communication phenomena; abstract characterizations of the nature of space; and the unexploited connectivity potential of emerging electronic network technology.