The theory of transitivity is an important part of Halliday's systemic functional grammar. It is also one of Halliday's most remarkable contributions to the study of grammar. However, the following discussion of the process types is not exactly in accordance with Halliday. What it provides is a logical classification of process types according to the general classes of things and their actions.

According to Halliday (1994: 106), a fundamental property of language is that it enables human beings to build a mental picture of reality, to make sense of their experience of what goes on around them and inside them". All these goings-on are sorted out in the grammar of the clause", so "the clause is ... a mode of reflection, of imposing orders on the endless variation and the flow of events". "The grammatical system by which this is achieved is TRANSITIVITY. The transitivity system construes the world of experience into a manageable set of PROCESS TYPES.

The term transitivity will probably be familiar as a way of distinguishing between verbs according to whether they have an Object or not. Here, however, it is being used in a much broader sense. As shown above, it refers to a system for describing the whole clause, rather than just the verb and its Object. However, it does share with the traditional use a focus on the verbal group, since it is the type of process that determines what participants are involved and how they

are labeled.

A process consists, in principle, of three components: the process proper, the participants in the process, and the circumstances associated with the process. This tripartite interpretation of processes is in accordance with Marxist philosophy, which maintains that the world is composed of entities that are in constant motion in particular circumstances. It also agrees with what lies behind the grammatical distinction of word classes into verbs, nouns and the rest. The process is typically expressed—or realized—by a verbal group in the clause, and is the central component of the message. The participants are normally realized by nominal groups while circumstances are typically realized by adverbial groups or prepositional phrases. Normally every process includes at least one participant, represented either by the Subject or by the Object; but circumstances are often optional, reflecting the background function of time, place, manner, cause, result, etc., in the clause. Therefore, the following discussion of the process types leaves out the circumstantial elements.

The concepts of process, participant and circumstance are semantic categories which explain in the most general way how phenomena of the real world are represented as linguistic structures. When particular types of processes are interpreted, more specific categories are adopted. Nevertheless, they all derive from and can be related to these three general categories.

According to Halliday, transitivity systems include six processes: material, mental, relational, behavioural, verbal and existential. However, his classification seems to be too general, so the boundaries between the process types appear to be too vague to be manageable. One example is the existential process. In Halliday's opinion, besides the typically used verb be, other verbs also occur in the process. "One group is a small set of closely related verbs meaning 'exist' or‘happen': exist, remain, arise; occur, come about, happen, take place. The other group embody some circumstantial feature; e.g. of time (follow, ensue), place (sit, stand, lie, hang, rise, stretch, emerge, grow). But a considerable number of other verbs can also be used in a range of abstract existential clauses; e.g. erupt, flourish, prevail." (Halliday, 1994: 142) (original, the author) However, these verb processes are not considered as existential in this book. They belong to other processes. Therefore, this book will offer a different classification of the process types according to the general categories of things and their actions. Altogether there are eight process types in the transitivity systems: material processes, abstract entity processes, sensuous processes, behavioural processes, mental processes, verbal processes, relational processes and existential processes. 

Though Halliday just talks about the transitivity systems in English, he hints that the interpretation of processes in terms of verbs, nouns, and the rest is "a pattern that in some form or other is probably universal among human languages" (Halliday, 1994: 108). Therefore, it is assumed that the following discussion of the process types is applicable to both English and Chinese.

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