In 1.2.4, we considered a simple picture of the operation of language. 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 and thus determine the proposition it expresses. Language is used to convey information by way of the propositions expressed by sentences. To have information is to be able to locate the actual world within some region in the space of all logically possible worlds. If I assert a sentence, I commit myself to its truth and thus to the actual world being one of the possibilities it leaves open; equivalently, I commit myself to the actual world not being one of the worlds it rules out. So someone may garner information from my assertion by accepting it as true and using the line it draws between the possibilities it leaves open and those it rules out to further pin down the location of the actual world.
Probably no one ever believed that things were quite that simple, but this picture or something like it was, until recently, the working model most logicians used for thinking about the function of language. Around 50 years ago, philosophers became interested in a number of features of language that suggest this picture is inadequate; and, in the last several of decades, these features have been incorporated into a number of richer models of language. The norms of deductive logic that we will study do not rest on the richer structure of these new models, so we will not consider them in detail. But some of the further features of language that they attempt to capture are intertwined with those we will study, so we need to take some time now to disentangle ourselves from a few of these features once and for all and to lay the groundwork for disentangling ourselves from others at later points in the course.
The complicating phenomena that we need to consider have come to be studied under the rubric of pragmatics. This term was originally introduced (by Charles Morris) as an alternative to semantics in order to distinguish issues concerning the relation between language and its users from the issues concerning the relation between language and what is spoken of. Its meaning is now less closely tied to this definition than to commonly agreed examples of pragmatic phenomena, including the following.
1) Sentences are not always used to convey information. When a sentence is used to convey information, the question of the truth value of information conveyed is a significant one. But not all sentences have truth values or raise questions of truth values. And even when a sentence does have a truth value, its truth value may be irrelevant for its use. Clear cases of a lack of truth value are questions and imperatives, but truth values may be beside the point for declarative sentences, too. The sentence I apologize for what I said is not said because it is true; rather, under the right circumstances, it is true because it is said. The moral: there are many ways of using sentences, many speech acts, besides assertion.
2) The information conveyed by a sentence (and thus its truth value) may vary with the context in which it is used. For example, there is no way to judge the truth value of a sentence like I put that here yesterday when it is taken out of context. This dependence on context is due to various phenomena known collectively as indexicality or deixis (some of which concern the role of the words I, that, here, and yesterday).
3) The information we derive from the use of sentences is not limited to the propositions they express. Further information is provided by the phenomena of implicature and presupposition (examples of which will be discussed below).
To disentangle ourselves from the variety of speech acts, it is enough to say that we are concerned only with assertion. So, when we consider sentences, we will limit ourselves to declarative sentences used assertively. Of course, there is a sense in which conclusions can be drawn from apologies and promises, but we will have a broad enough range of study without attempting to deal with such reasoning. Moreover, many accounts of speech acts other than assertion treat propositions as important components of their meaning, and this gives the logic of assertions a central place in the logic of all speech acts.
While this is enough to settle the relation of our study of logic to the variety of speech acts, the other two sorts of complication will require more detailed consideration.