Introduction
The acquisition of knowledge about a task can be viewed as a process of incorporating new knowledge into some existing knowledge structure [Rosenbloom 1988]. The existing knowledge can guide and constrain the search for new knowledge, and the process of integrating the new knowledge with the old may identify additional opportunities for learning. An acquisition system that takes this view needs to represent and understand the knowledge about the task as well as the process of finding and integrating new knowledge.
A general trend in research on knowledge acquisition has been to make the knowledge structures that guide acquisition increasingly explicit. In early acquisition tools many of the requirements that needed to be satisfied when adding a new piece of knowledge were not stated. The result was that they could not provide very precise guidance for acquisition. Later, more of these requirements were made explicit, but they were embedded within the acquisition tools themselves. This allowed the tools to provide more specific guidance, but the requirements that were embedded in the tools could not be changed, which meant that some important aspects of the knowledge-based system being built could not be changed. More recent work has focused on making these requirements explicit and represented outside of the tool itself. A result of this has been more flexible acquisition tools that allow users to make a greater variety of changes to the knowledge-based system being built.