 A conversation is a loosely collaborative exchange of information having one or more mutually understood aims. Participants presume they share certain pertinent information already, but each is presumed to need further information the others might be in a position to provide. In conversations we can and regularly do hold the other participants responsible for the truth of both more and less—in some ways more, in other ways less—than they explicitly put into words. For once a conversation is under way, each of us is entitled to interpret the words of the others in light of the supposition that they are cooperative, rational, and unconfused about the current state of conversational play, even if there is good reason to doubt or disbelieve this supposition, e.g. even if they exhibit troubling signs of recklessness or deceitfulness. This is an application to speech of a much more general principle: in improvised voluntary collaborations of any sort, the responsibilities a given party shoulders are those he willingly lets himself seem ready to shoulder, no more and no less.