PEP Inferred from the Validities of Some Characteristic Formulas in Modal Logic
换位原理与几个模态特征公式的有效性

Position-Exchange-Principle(PEP) is described as an axiom scheme in Reasoning About Others(RAO) and regarded as a basic rule for agents to reason about knowledge of others in a multi-agent system. In this paper, relationships between some characteristic formulas in normal modal logic with their frames are discussed. It has been discovered that validities of the modal formulas are conditional even though some of them are intuitively valid. Finally, validities of two formulas of PEP proposed by papers 1 to 3 are proved by using of modal logic and Kripke's semantics of possible worlds.