well, but we choose to focus our investigation on one important use. This focus results in consideration of only part of the story about the contents and function of commonsense knowledge. This is a drawback to our approach, but it also affords an advantage. By looking at one type of use of commonsense knowledge, we can characterize the constraints on its use and the function it must serve. For example. one characteristic of human knowledge about object types is that this knowledge is typically organized within a hierarchical structure. As we proceed in our discussion of commonsense planning we will appreciate the functional utility of this organization in the planning process.

     What is meant by constraints here? There are really two types. The first are the relatively invariant constraints imposed by the processing advantages and limitations of the human mind that created this knowledge for its use. Some of the advantages that the mind seems to enjoy are the ability to retain and efficiently retrieve a great deal of general and specific knowledge about the everyday world, and the use of an extremely flexible control structure. The mind's disadvantages seem to be related primarily to a rather limited working memory, and some unreliability in the ability to retrieve specific facts that were known at some point in time.

     The second type of constraints that affect the use of commonsense knowledge arise from the world in which humans use this knowledge. On the one hand, our everyday world is an open one. Here nature and others like ourselves create changes quite without notice of or regard to our own current plans and goals. Consequently, we rarely have the luxury of complete knowledge. These changes that we could not foresee may simplify or complicate the plan we had in mind. In either case, a revision of the plan is required. A system that must plan in such an open world must, therefore, attempt to create plans that are easily revised in light of changing circumstances. On the other hand, the everyday world is also a world of convention and standard practice. Artifacts are manufactured with specific purposes in mind and generally the manufacturers of these artifacts are motivated to work out for us some of the interactions that exist between the various artifacts required for many of the tasks we must perform. For example, screwdrivers are made for the different types of screws, which in turn are created to function effectively in different types of material and under differing conditions of required torque. The existence of convention does not guarantee adherence to these conventions. But, again, a planning system that must function in such a world is expected to take advantage of whatever aid the existence of such conventions can give to the planning process.

     Consideration of both the internal and external constraints on commonsense planning coupled with an examination of examples of this reasoning alert us to the role, or function, that commonsense knowledge serves. As someone trained in psychology, the biological heuristic that structure and function are often related, seems also to be a valid heuristic for studying commonsense knowledge-itself the product of a natural intelligence. This relation between structure and function in human cognition is probably best documented in the case of linguistic abilities (Lenneberg, 1967). There is no reason to suspect that such relations are confined to

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