2.1.1 The Definition of Logical Connectors

In fact, different linguists have different definitions on logical connectors. Biber et al. used the term linking adverbials, which can state the writers/speakers viewpoint of relationship between two parts of discourse (Biber et al., 2000: 970). Oshima and Hogue defined linking adverbials as words and phrases that connect the idea in one sentence or clause with the idea in another (Oshima & Hogue, 1997: 127). Halliday and Hasan called them "conjunctions", which are words or short phrases showing the interrelation between two statements or clauses in a text by explicitly specifying a conceptual relationship between statements (Halliday & Hasan, 1976: 226). Logical connectors are used to show or express logical relations, but those obvious logical relations need not logical connectors to show them clearly.

2.1.2 The Classification of Logical Connectors

There are five major devises to achieve semantic cohesion in the book Cohesion in English (Halliday & Hasan, 1976: 226). It is realized through the semantic connections between differences forms in a discourse, which is reference, substitution, ellipsis, conjunction and lexical cohesion. Conjunction is a different type of semantic relation from the other cohesive devices. With these conjunction elements, one can comprehend the semantic relations between sentences. He can even logically presume the meaning of the following sentence by virtue of what has gone. According to their semantic relations, Halliday and Hasan pided logical connectors into four types: (Halliday & Hasan, 1976: 226)

Additive means by that a linking word or phrase used to add new content to express previous meaning when a previous sentence is finished. For example, and, or, nor, furthermore, alternatively, incidentally, that is, for instance, likewise, on the other hand, etc. 

Adversative indicates that the linking word or phrase connect two the meaning of opposite sentences. Examples of adversative relations are: yet, though, but, however, actually, instead, at least, rather, in any case, anyhow. 

Causal refers to a word or phrase connects two sentences that relationship is cause-effect or antecedent-consequent. Such as so, hence, consequently, because of this, for this reason, as a result, for this purpose, reversed form such as for, because, then, in that case, under the circumstances, otherwise, in this respect. 

Temporal logical connectors usually used to express the temporal order of things or actions happened. The relations of temporal could be pided into several subcategories, which are sequential ones, simultaneous ones, preceding ones and conclusive relations, such as at the same time, simultaneously, in the end and so on. 

In the book of A Comprehensive Grammar of English (Quirk et al., 1985: 631), it has made an extensive accept for classifying logical connectors. They, according to the semantic relations, classify logical connectors into seven major categories:

1. Listing. Listing logical connectors can be subpided into enumerative ones and additive ones, such as first(ly), on the other hand, similarly, furthermore, moreover.

2. Summative. These types of logical connectors "introduce an item that embraces the preceding ones, such as therefore, then, thus, to sum up, in conclusion.

3. Appositional. These group logical connectors either express the content of the preceding item or items in other terms, for example, namely, for instance, that is to say, specifically.

4. Resultive. These logical connectors need reasons or causes to be given in the preceding discourse, for example, accordingly, consequently, hence, as a consequence, somehow.

5. Inferential. Inferential logical connectors state a conclusion based on logic and supposition (Quirk et al., 1985: 638), such as else, otherwise, in other words, in that case.