Since sending explicit messages can be costly, people often utilize “what is not said,” i.e., informative silence, to economize communication. This paper studies the efficient communication rule, which is fully informative while minimizing the use of explicit messages, in cooperative environments. It is shown that when the notion of context is defined as the finest mutually self-evident event that contains the current state, the efficient use of informative silence exhibits the defining property of indexicals in natural languages. While the efficient (...) use of silence could be complex, it is also found that the efficient use of silence can be as “simple” as the use of indexicals in natural languages if and only if the information structure satisfies some centrality and dominance properties. (shrink)