From now on I will use the term `arguments' to refer to obligatory constituents, those that are required by the predicate. Facultative constituents will be called `satellites'. Arguments and satellites together with the predicate from a `predication'. In a `predication' I will make a distinction between a `nuclear predication', or `nucleus', on the one hand, and a `periphery', on the other. The nuclear predication consists of the predicate and its arguments. The periphery consists of one or more satellites. Within the nuclear predication arguments have a particular semantic relationship with respect to the predicate (see crosssection 2.3.), or, to put it in another way, they have a particular `semantic function'. We see the role of Agent in pater and that of Patient in filium in example (1). Which semantic functions may occur with a specific predicate depends on the meaning of that predicate. The arguments not only fulfil a specific semantic function, they also have a syntactic function, as we have already seen. Thus, pater in (1) has the syntactic function Subject in this sentence. The syntactic functions of arguments will be discussed in some detail in crosssection 2.2.
Turning to satellites now, we have already noted a distinction between two types. The first type consists of those satellites that specify the nuclear predication, indicating, for example, the Manner in which a certain action is performed. These satellites will be said to have the syntactic function `Adjunct'; in Latin grammars they are usually called `adverbials'. Adjuncts
will be discussed more fully in chapter 3. The second type of satellites consists of those which contain information about the whole of the nuclear predication and its Adjuncts. They will be called `Disjuncts' or `sentence adverbials'. Disjuncts and the semantic functions they may fulfil will be dealt with in chapter 4. The distinctions made so far can be represented graphically, as in figure 1.1. 
Constituents in a sentence have been shown to fulfil both a syntactic and a semantic function. They may also fulfil a specific `pragmatic' function. Pragmatic functions are a relatively recent addition to the description of sentence structures, and the incorporation of pragmatic information in linguistic analysis has not yet found general acceptance among specialists. However, it will become clear further on in this book that a number of characteristics of Latin cannot be described in an adequate way if we do not take into account the particular communicative setting in which sentences appear. Therefore, it is useful at this point to introduce a few notions, viz. that of `Topic' and that of `Focus'. Consider the following two sentences:
(9) Where did you buy that hat, John?
(10) I bought it/that hat at the flea market
In (10) the constituent at the flea market is the requested, new, and therefore relatively `salient', information with respect to John's buying a hat. We will say that the constituent at the flea market has the pragmatic function `Focus'. The hat, on the other hand, is `given' information and it is the entity about which sentence (10) presents new information. The constituent about which something is predicated henceforth will be called the `Topic' of the sentence.
A further example of `Focus' can be found in (11), which provides another possible answer to the question in (9):
(11) I didn't buy it, I found itIn this sentence found replaces bought or, in other words, is contrastive with respect to bought. As a matter of fact, we will see that it is useful to make a distinction between several types of Focus. In Latin Topic constituents can often be omitted in context. Focus constituents may be marked by certain intensifying particles like et … et, quidem and sane.  In chapter 9, which concerns word order, pragmatic functions will be shown to be extremely important for explaining the order of constituents in Latin sentences and phrases. However, pragmatic notions will be shown to be relevant to other sections of Latin syntax as well.  See chapter 4 for the pragmatic function `Theme'. 
Pinkster, Harm (1942-) , Latin Syntax and Semantics [info], xii, 320 p.: ill.; 24 cm. [word count] [Pinkster].