Default assumption: Plastic anchors are usually sold with several in a packet and unless all are used, some will remain. Subgoal: Get the plastic anchor. Default assumption: Tools and hardware are typically kept in the basement in the vicinity of the work bench. Subgoal: Obtain a screw of appropriate diameter and length. Default assumption: Box filled with miscellaneous screws and nails typically will contain a screw that will be appropriate for use in a plastic anchor. Subgoal: Get the screw. Default assumption: Tools and hardware are typically kept in the basement in the vicinity of the work bench. Subgoal: Make a hole in the wall for the anchor. Subgoal: Get one of the hammers. Default assumption: Tools and hardware are typically kept in the basement in the vicinity of the work bench. Subgoal: Get the awl.

    The example could be continued a bit more to include: determining whether I have picture wire; actions of going to the basement and bringing tools, supports and picture wire to the living room; making the hole; and so on. However, the plan would probably not be worked out in much more detail before I decided that I had planned enough to support the guidance of the actual execution of the plan.

     Seemingly simple and certainly mundane scenarios such as these have kept my colleagues and I thinking and wondering about how to represent and use commonsense knowledge in planning and plan recognition for longer than I like to admit (Schmidt, Sridharan, & Goodson, 1978). I suspect that there are enough issues of logic and reasoning lurking in examples such as these to keep many of us busy for some time to come.

     But although I have no definitive answers to share with you in this paper, we have implemented a planning system that can mimic some aspects of the reasoning that underlies this example. By relating here some of the details about how knowledge is structured and used in this system, I hope to contribute to the collective task of deepening our understanding of commonsense reasoning.

2 A Digression into Methodology

Before proceeding with this discussion, a few remarks about the way in which we have approached the task of understanding commonsense reasoning should be made. The foregoing example to which we will soon return illustrates one aspect of the approach. The starting point involves a consideration of how commonsense knowledge is used by the holder and creator of that knowledge, namely, the human information processing system. In this case, the use is for planning and supporting the execution of actions. The same knowledge may be put to use in other ways as

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