Conventional knowledge bases comprise essentially static specifications of some model. Traditional logic methods are quite adequate for reasoning within such systems, since the model’s assertions are monotonic and therefore unchanging. There are two main limitations to this approach: (i) knowledge about the (ever-changing) world must reflect its ever-changing nature and (ii) the symbols and relationships within a model must correspond to individuals, objects, and relationships in the world. We assume that there are at least three broad purposes in developing and

maintaining a knowledge-based computer system.