Semioticians think that metadiscourse should be considered as a sign. For those people, metadiscourse is the semiotic interpretation of the discourse or text. Philosophers gave propositional logic as the definition of metadiscourse, emphasizing logical structures and logical problems. They separate language into metalanguage (used to refer to language) and object language (used to refer to the reality). However, Crismore believes that this definition is too limited, because it considers object language as consisting of only propositional content and, accordingly, metalanguage is excluded from the language domain. Speech communication theorists regard metadiscourse as metacommunication, which means communication about communication, whether it is verbal or nonverbal and whether it is about communication in general or about some specific communicative interactions. Metadiscourse is also broadly defined as commentary in the field of rhetoric. Some rhetoricians think that commentary is text about text, while some define metadiscourse as figures of thought, which are rhetorical strategies used for effective communication.

Linguists, on the other side, define metadiscourse more narrowly. Keller (1979) considers metadiscourse as “gambits”, which are psychological strategies used by participants in communication to introduce what to say next. Sociolinguist Schriffrin (1980) discusses “meta-talk” which means the talk about the ongoing talk. Beaivaos defines metadiscourse as “the elements in a sentence that convey illocutionary content in either fully or partially explicit form” in his speech (1989).

Some early researchers preferred the distinction between propositional discourse and metadiscourse, but some later ones opposed it. For example, Williams (1981) and Dillon (1981) present different planes of meaning. That is, the propositional level of discourse supplies the reader with information about a topic, and the metadiscourse level calls attention to the act of writing. This is also Vande Kopple’s view (1985 and 2002). He agrees with them and expresses his opinion most recently like this:

On one level we expand ideational material. On the levels of metadiscourse, we do not expand ideational material but help our readers connect, organize, interpret, evaluate, and develop attitudes towards that material (Vande Kopple, 2002).

However, how metadiscourse can constitute a different level of meaning is difficult to see. It is certainly possible that the easiness for people to distinguish the propositional content of a text from the particular way in which it is expressed, for even the distinctive reading positions we might adopt are constrained by the text and the culture of a community of readers. But this does not imply that metadiscourse can be omitted from a text without changing its meaning (Hyland and Tse, 2004).

Generally speaking, meaning is not the same as content. It depends on all the components of a text. Propositional and metadiscoursal elements always occur together, generally in the same sentences. It is not surprising that a stretch of discourse may have both functions. Such kind of integration is quite common for each element to express its own content. That is, one is concerned with the world and the other with the text and its reception.

2.2 Classifications of Metadiscourse
There are various classifications of metadiscourse according to different standpoints. Most of them are influenced by Halliday’s theory of metafunctions of language. In this section, the classifications of Crismore (1983) and Hyland (2004) will be reviewed. Those two people are the most famous and representative scholars in the field. The classification of Crismore (1983) represents category in earlier times, and the classification of Hyland (2004) is put forward about 20 years after Crismore’s 《生活大爆炸》与《成长烦恼》元话语运用比较(3):http://www.youerw.com/yingyu/lunwen_3844.html