Persuasive presuppositions

A recurrent claim, coming from different approaches to pragmatics, argumentation theory and related disciplines, is that informative presuppositions have a special persuasive force. My aim in this paper is threefold: first, it is to achieve a better understanding of this claim. I argue that we should carefully distinguish between persuasion, which involves forming a belief, and accommodation, which requires acceptance of a proposition for the purpose of the conversation. Consequently, we should distinguish between the persuasive force of presuppositions and a different claim concerning our tendency to accommodate presuppositions.

A presupposition is an implication of a certain kind, that is usually identified by possessing one or more of the following features. First, presupposition is indirect or less explicit, in contrast to assertion, which is direct and explicit (Pagin and Marsili, 2021). It is not foregrounded, that is, not part of the main assertion, but instead backgrounded (Stalnaker, 1973: 198, Levinson, 1983: 179, Potts, 2015: 35) or not-at-issue content (Simons et al., 2010). Second, a presupposition is a proposition that the speaker takes for granted and treats as uncontroversial.1 A third feature is projection: presuppositions are preserved under embedding the initial sentence in a variety of linguistic contexts, such as negation (Frege, 1892: 40), modal operators, conditional ‘if’, and questions.2 Jointly, these characteristics of presupposition help us distinguish them from the asserted content of an utterance of a sentence (which lacks the three characteristics), from conversational implicatures (which are not taken for granted and do not project) and from other implications of an utterance of a sentence.

Stalnaker explains the phenomenon of presupposition in pragmatic terms, on the basis of “general assumptions about rational strategy in situations where people exchange information or conduct argument” (Stalnaker, 1974: 205).3 One important notion in this approach is that of common belief: a proposition p is common belief in a group if and only if (almost) everybody in that group believes that p, and believes that (almost) everybody believes that p etc. Next, acceptance is defined as the mental attitude of treating a proposition as true for some purpose. Finally, the definition of the common ground of the group (or CG), is the following:

 Def1: CG(p) iff it is a common belief in the group that everybody accepts p. Speaker presupposition is defined as a propositional attitude, as follows:

 Def2: The speaker presupposes p (relative to the group) iff the speaker believes that CG(p).

That is, a speaker presupposes p iff she believes that p is part of the CG of the conversation, which means that she believes that everybody in the group believes that everybody believes etc. that everybody accepts p. This definition of speaker presupposition captures the intuitive sense in which we say that the speaker takes for granted what is presupposed: taking for granted that p is precisely believing that p is part of the CG.

Stalnaker's definition of speaker presupposition in Def2 is the definition of a propositional attitude. It not a linguistic phenomenon and is not related to any particular utterance of a sentence. The related linguistic phenomenon is that the correct use of certain sentences requires that the speaker presuppose a particular proposition. The definition of sentence presupposition introduces this normative notion:

Def3: Sentence s presupposes p iff the use of s would for some reason be inappropriate unless the speaker presupposes p.

From the definition of speaker presupposition (Def1) and that of common ground (Def2), it follows that sentence s presupposes p iff the use of s would for some reason be inappropriate unless the speaker believes that CG(p). That is, the use of a particular sentence is appropriate if only if the presuppositions of that sentence are part of the CG. But, given that the evaluation of the speech act takes place at a time after the time of the utterance (Stalnaker, 2014: 50), a sentence that triggers a presupposition p might be appropriate to utter even if p is not part of CG at the time of the utterance, but instead constitutes new information to the audience. Still, the utterance is appropriate if the participants in the conversation are willing to add p to the CG when the speech act is manifestly recognized. This phenomenon is called presupposition accommodation. (David Lewis 1979: 347) called attention to it, writing that “presupposition evolves according to a rule of accommodation specifying that any presuppositions that are required by what is said straightway come into existence, provided that nobody objects.” Thus, an utterance of sentence (1) is judged as appropriate even if the proposition that Sarah did not know how to swim before is not part of the CG at the moment of the utterance, but is added at some point after the utterance is made. This is achieved automatically, as long as no one objects to the proposition.