ABSTRACT
Common knowledge is widely used in economic theory, theoretical computer science, linguistics, and philosophy. This chapter considers an influential argument in favor of the position. It focuses on conceptual issues related to the Default Position. While the various notions of common knowledge are conceptually distinct, the default position in the literature has been that they coincide. Although the arguments have been taken by many authors to motivate the idea that common knowledge is at least importantly related to public information, some of the same authors have also been suspicious of the idea that public information requires common knowledge itself. The Default Position is the benchmark position in the literature; it is almost invariably the starting point of discussions about common knowledge and public information. In philosophy, perhaps the most common lesson that has been drawn from the surprising result is that common knowledge is important for successful coordination.
