Planning in, or reasoning about, the world given incomplete knowledge thus presents a number of challenges. In some cases, it is possible and sufficient to identify likely states, and reason about them, assuming that mistakes either don't matter or can be corrected as they are discovered. In other situations, it is necessary to explicitly develop solutions that are appropriate for any of the different ways in which things may actually be, thus ensuring that those solutions are robust.
We have also been interested in nonmonotonic reasoning, which addresses how to arrive at reasonable default conclusions to fill knowledge gaps in reasonable ways, but lately that interest has been focused mainly on computational, rather than representational, issues.