This takes care of the types of example which Strawson and Schiffer used to show that, in order to communicate, it is not quite enough to inform an audience of one’s informative intention. For instance, in the example in an earlier section, Mary leaves the pieces of her broken hair-drier lying around, intending thereby to inform Peter that she would like him to mend it. She wants this informative intention to be manifest to Peter, but at the same time, she does not want it to be ‘overt’. In our terms, she does not want her informative intention to be mutu-ally manifest. Intuitively, what she does is not quite communicate. Our redefinition of a communicative intention accounts for this intuition. What difference does it make whether an informative intention is merely manifest to the audience or mutually manifest to audience and communicator? Should this really be a criterion for distinguishing communication from other forms of information transmission? Is it more than a technicality designed to take care of implausible borderline cases dreamed up by philosophers? Our answer is that there is indeed an essential difference. Consider first a more general question: why should someone who has an in-formative intention bother to make it known to her audience that she has this intention? In other words, what are the reasons for engaging in ostensive com-munication? Grice discussed only one of these reasons: sometimes, making one’s informative intention known is the best way, or the only way, of fulfilling it. We have shown that people sometimes engage in ostensive communication even though the informative intention could be fulfilled without being made manifest: for exam-ple, by providing direct evidence for the information to be conveyed. However, even in these cases, ostension helps focus the attention of the audience on the rel-evant information, and thus contributes to the fulfilment of the informative inten-tion. This is still the Gricean reason for engaging in communication, just slightly extended in scope. However, we want to argue that there is another major reason for engaging in ostensive communication, apart from helping to fulfil an informative intention. Mere informing alters the cognitive environment of the audience. Communication alters the mutual cognitive environment of the audience and communicator. Mutual manifestness may be of little cognitive importance, but it is of crucial social impor-tance. A change in the mutual cognitive environment of two people is a change in their possibilities of interaction (and, in particular, in their possibilities of fur-ther communication). Recall, for instance, the case of Peter leaning back to let Mary see William coming their way. If, as a result of his behaviour, it becomes mutually manifest to them that William is coming, that they are in danger of being bored by his conversation, and so on, then they are in a position to act efficiently: i.e. promptly. All Mary may have to do is say, ‘Let’s go!’; she can feel confident that Peter will understand her reasons, and, if he shares them, will be ready to act without ques-tion or delay. In the case of the broken hair-drier, if Mary had made mutually manifest her wish that Peter would mend it, one of two things would have happened. Either he would have mended it, thus granting her wish and possibly putting her in his