We present a representation for reasoning and planning with an incomplete state description (open-world) called PSIPLAN-S. The presented formalism has several properties critical for application domains with a large degree of incompleteness in the state description, particularly, in domains with a large or unknown set of all objects. The formalism offers (1) considerably expressive state and goal description language, that includes limited universal quantification, (2) representation of sensing

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... actions and knowledge goals, (3) a correct and complete state update procedure, and (4) complete reasoning within a substantial subset of the language. The approach is illustrated by examples from a working system. KEYWORDS: automated planning, planning with incomplete information and sensing, reasoning about actions. denotes all ground instances of the formula Kn(¬PrefBib(b)) minus two exceptions: Kn(¬PrefBib(ACM )) and Kn(¬PrefBib(RI )), and could also be expressed as a universally quantified formula PSIPLAN-S uses a combination of ground atoms and ψ-forms to represent incomplete information. For example, to represent statement ST below ST: The only preferred bibliographies are the digital library of the ACM and possibly the ResearchIndex database.