1.3.1. A model of language

The idea of truth conditions or of a proposition suggests a simple picture of the way language works. According to this picture, each sentence has truth conditions that are determined by the semantic rules of the language. These truth conditions settle the truth value of the sentence in each possible world, something that is encapsulated in a proposition. The proposition expressed by a sentence is its meaning. The meaning of an expression smaller than a sentence is to be found in the contribution this expression makes to the propositions expressed by sentences containing it. From this point of view, the function of language is to convey propositions.

Just as the information content of a sentence is to be found by considering the range of possible worlds it rules out, the information that a person possesses is to be found by considering the possible worlds that he or she is able to rule out. The more you can rule out, the more information you have; and the kind of information you have is determined by the particular worlds you can rule out. This means that the sum total of your knowledge can be thought of as a proposition.

Anyone’s aim in acquiring information could be described as an attempt to distinguish the actual state of the world among the various alternative possibilities—in short, to locate the actual world within the space of all possible worlds. The proposition representing your knowledge goes some distance towards in ruling out some possibilities. But it will leave many open, and the actual world could be any of those open possibilities. If someone conveys a proposition to you and you accept it, you are able to rule out a whole region of logical space, a region that can be added to the region ruled out by your existing knowledge. And, in general, this will reduce your uncertainty about the location in logical space of the actual world.

You can generate information to give to others by delimiting a region within the total area you know to be ruled out. Ideally, perhaps, you would simply convey the whole of what you know; but language limits your ability to do this since only a limited range of propositions are expressed by reasonably short sentences. To convey information, you select a sentence that is entailed by what you know and assert it, thereby conveying the proposition this sentence expresses.

This is process is illustrated in the following artificial example of sharing information. Figure 1.3.1-1 animates a conversation between two talking heads, each with a thought bubble indicating the content of that person’s beliefs.

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Fig. 1.3.1-1. An animation of a conversation in which information is shared. The button > will play the full conversation while the buttons φ, ψ, χ, and θ will each play one of its four stages. The buttons |< and >| move to the initial and final state, respectively.

Initially, the person on the left is able to rule out regions at the left and right of logical space as possibilities for the actual world while the person on the right is able to rule out regions at the top and bottom. The animation then shows a conversation in which each party in turn notices the truth of the one the sentences φ, ψ, χ, and θ and asserts it. The other person accepts this assertion as true and adds its content to the region ruled out by his or her beliefs, so the shared content increases over the course of the conversation. At the end, each is able to locate the actual state of the world as being somewhere in the middle of the full range of possibilities though the shapes of the areas they leave open still differ.

In this conversation, each party is depicted as accepting what the other says as true and adding it to his or her own beliefs. The person accepting the assertion could be said to modify his or her beliefs in a way that makes it something he or she might assert. This is an example of a process that the philosopher David Lewis labeled accommodation. In this case of accommodation, one’s beliefs are altered to accommodate an assertion someone else has made.

Of course, we do not always accept what others say—i.e., we do not always alter our beliefs to accommodate their assertions—for we may doubt that they are sincere or that they know what they are talking about. But this cannot be the ordinary case. Words can acquire and maintain a conventional meaning only if people usually mean what they say. And the act of asserting a sentence could not have the significance it does unless people were usually willing to accept assertions as well-founded. A critical attitude is important; but, at least practically, it must be the exception. Even when we are critical and ask for the grounds of someone's assertion, our request can be met only if we are at some point willing to accept the grounds we are offered; and, when we do accept these grounds, it will rarely be because there is no room for further doubt. In short, while we do not always accommodate what others say, accommodation is central to the aspects of language this model captures. And the reasons that it is central suggest it will be important for any sort of shared understanding.