7.6 The principle of relevance

　 Speech act theory and cooperative principle seriously challenge the code model of human communication, which has existed in western philosophy since Aristotle. According to the code modal, communication is achieved by encoding and decoding messages. It has been shown in the previous sections that illocutions and implicatures are not directly represented by linguistic codes. They must be inferred. Philosophers and pragmatists who explore this aspect of communication proposed the inferential model. According to this model, communication is achieved by producing and interpreting evidence. Sperber and Wilson (1986) attempt to show that verbal communication involves both coding and inferential processes. They put forward the principle of relevance to account for how communication is achieved. They argue that the principle of relevance is essential to explaining human communication:

Human cognitive processes ... are geared to achieving the greatest possible cognitive effect for the smallest possible processing effect. To achieve this, the individual must focus his attention on what seems to him to be the most relevant information available. To communicate is to claim an individual's attention: hence to communicate is to imply that the information communicated is relevant (Sperber and Wilson 1986: vii).

　 Relevance is a relative notion. It is determined by two factors: contextual effect and processing effort. The greater the contextual effect, the greater the relevance. Processing effort is a negative factor: the greater the processing effort, the lower the relevance. The relation between relevance and the two factors can be represented by a fraction:

　In verbal communication, the speaker must attract the hearer's attention. To succeed, the communicator necessarily chooses the most relevant utterance from a range at his/her disposal that fits best to the context, the one that will call for the least processing effort. This is summarized as the principle of relevance:

　 Every act of ostensive [-inferential] communication communicates the presumption of its own optimal relevance ( Sperber and Wilson 1986: 158)

　 The term ostensive [-inferential] expresses the view that communication involves both coding and inferring, a view of communication that amalgamates the code model and the inferential model. Let us use an example to illustrate what the principle means.

　　(35) Peter: Do you want a cup of coffee?
　　　　 Helen: Coffee would keep me awake.

Imagine that the context is this: Peter and Helen are a couple; they have just arrived home after a long trip; Peter finds Helen very weary. Under the circumstance, a range of utterances are possible for Peter:

　　(36) You are very tired. I wonder whether you want a cup of coffee.
　　(37) You are very tired. A cup of coffee will refresh you. Shall I get you a cup of coffee?
　　(38) Do you want a cup of coffee?
　　(39) Do you want to watch TV?

The list is not exhaustive, of course. Among these, (36), (37) and (38) are obviously more relevant than (39), as when physically tired, entertainment is not the first thing people need. Among the three, (38) is the most relevant because it requires the least processing effort. For Helen, there are also a range of options:

　　(40) I like coffee but I don't want it now, because I want to sleep.
　　(41) I want to sleep, but I have to stay awake to prepare for tomorrow's test, so I do need a cup of coffee now.
　　(42) Coffee would keep me awake.

Literally, (42) doesn't reply Peter's question. However, given the fact that Peter knows whether Helen has to stay awake late, (42) is of more optional relevance than (40) or (41). Helen is sure that Peter will understand the implicature through inference.

　 Sperber and Wilson's theory further develops the idea of implicature as proposed by Paul Grice. According to Gricean theory of cooperation and implicature, violation of the maxims produces implicature. The theory of relevance accounts for the fact that implicature and cooperation are not incompatible. This is one of the merits of the theory that make it influential in cognitive exploration into language use.