Normally, we can recognize a text as a sentence or a group of sentences because we can see a clear relationship of ideas unfolding. Yet how can we identify what makes a text cohere? What differentiates a cohesive grammatical unit from a random collection of sentences? Halliday and Hasan set out to clarify how the two differ.
They begin by defining text as "not just a string of sentences·.It is not simply a large grammatical unit, something of the same kind as a sentence, but differing from it in size--a sort of supersentence·a semantic unit" (291). Because it is a semantic unit, its texture (the state of being a text) is dictated by its interpretation within a particular context, or environment. Thus, it can be either spoken or written and of any length. For example:
[I:I] Wash and core six cooking apples. Put them into a fireproof dish.
This text is coherent because them in the second sentence clearly refers back to apples in the first. By itself, either sentence would not mean the same as they do together, and, in fact, the second sentence without its contextual reference would not make much sense.
While a text may be of any length, it is normally longer than one sentence. Such things as public notices and slogans, however, may be only a short phrase:
[7:1] a. No smoking
b. Site of an early chapel
Cohesive relationships between words and sentences have certain definable qualities that allow us to recognize the supersentence. These are: reference, substitution, ellipsis, and conjunction. One of the reasons that this book is so useful is that Halliday and Hasan provide a very detailed explanation and examples for each of these; here I will give a brief summary.
Reference
There are also times when the meaning is not explicit from the text itself, but is obvious to those in a particular situation. This is called exophoric reference.
[2:2] For he's a jolly good fellow
And so say all of us.
As readers outside of this environment, we are unfamiliar with who the he is that is being referred to, but, most likely, the people involved are aware of the he, and therefore, can find texture in the sentences.
Substitution
Substitution is somewhat different from reference in that another word
takes the place of the thing that is being discussed. Where reference is
a relation between meanings, substitution is a grammatical relationship.
There are three general way to substitute in a sentence: nominal, verbal,
and clausal. The pronoun one is often used in nominal references,
as it is in the following example:
[3:17] b. Let's go and see the bears. The polar ones are over on that rock.
In this sentence, ones is taking the place of bears in the previous sentence. Verbal and clausal substitution function in the same way, replacing the verb and the clause respectively in the preceding sentence.
Ellipsis
If substitution is replacing one word with another, ellipsis is the
absence of that word, "something left unsaid" (142).
While many sentences presuppose some prior knowledge by its audience, ellipsis
requires retrieving specific information from preceding information that
can be found in the text.
[4:2] Joan brought some carnations, and Catherine some sweet peas.
There is only one possible interpretation: Catherine bought some sweet peas.
Conjunction
The last of the three relationships, conjunction, functions somewhat
differently than the other three because it is not strictly semantic or
anaphoric, rather, it is related to the entire environment of a text. The
conjuntive elements "presuppose the presence of other components in the
discourse" (226). Instead of giving cohesion to a
text, it actually coheres two sentences together. And is the simplest example:
[5:9] 'While you're refreshing yourself,' said the Queen, 'I'll just take the measurements.' And she took a ribbon out of her pocket, marked in inches·
Although there is a distinct shift from one sentence to the next, the
sentences are still very much a part of a coherent text.
In this book, Halliday and Hasan have, basically, provided some solid heuristics for what makes a text a text. It is further useful because of its comprehensive list of principles, coding schemes, and sample texts given at the end. As a whole, it answers to question of 'what makes a text cohere' by giving its readers a clearer understanding of the general principles of texture.