 Aumann (1976) gives a different characterization of common knowledge which gives another simple algorithm for determining what information is commonly known. Aumann’s original account assumes that the each agent’s possibility set forms a private information partition of the space \(\Omega\) of possible worlds. Aumann shows that a proposition C is common knowledge if, and only if, C contains a cell of the meet of the agents’ partitions. One way to compute the meet \(\mathcal{M}\) of the partitions \(\mathcal{H}_i, i \in N\) is to use the idea of “reachability”.