 There are different ways to make this rough idea precise (see the entry on  convention).  According to one important view, a sentence S expresses the proposition p if and only if the following three conditions are satisfied: (1) speakers typically utter S only if they believe p and typically come to believe p upon hearing S, (2) members of the community believe that (1) is true, and (3) the fact that members of the community believe that (1) is true, and believe that other members of the community believe that (1) is true, gives them a good reason to go on acting so as to make (1) true. (This a simplified version of the theory defended in David Lewis 1975.)