Assertion and the Set of Live Possibilities

This episode turns from possible worlds to the speech act that changes which of them remain live in a conversation: assertion. Robert Stalnaker’s 1978 essay “Assertion” treats a conversational context as a set of possibilities compatible with what the participants take for granted. An accepted assertion narrows that set. 1

The episode distinguishes the common ground—the propositions treated as shared background—from the context set, the possible worlds in which those propositions are true. This makes an assertion more than a sentence with a truth value: in the right conversational conditions, it is a proposal to remove possibilities from the public field. 2

The model is deliberately public and modest. A proposition may enter the common ground for the purposes of the conversation without becoming a private belief in every participant’s head. The point is coordination: speakers need a manageable record of what they are treating as settled in order to continue reasoning together. 3

Stalnaker’s model imposes three pragmatic constraints on an assertion. The asserted proposition must be true in some, but not all, of the worlds still compatible with the context. The utterance must express a proposition relative to each context-world, with a truth value in each one. And it must express the same proposition throughout the context set. 2

Together, the constraints explain why an assertion cannot simply repeat what everyone already takes for granted, contradict the conversational background, or shift its content as the context changes. They do not yet provide a complete theory of everything that makes assertion distinct from assuming or conjecturing. They provide a clean model of the public update an assertion is supposed to achieve. 2

The episode ends with the remaining question: if the formal update only tracks which possibilities survive, what else must a theory of assertion capture—the speaker’s commitment, the listener’s entitlement to rely on the claim, or the way the proposition was presented?

The primary publication record for “Assertion” appears in Robert Stalnaker’s bibliography, alongside his later work on Common Ground, Context and Content, and Inquiry. 1