In comparison, other linguists approach modality from different angles.Perkins (1983: 1-12）inherits Aristotle’s idea, holding that modality is related to such notions as necessity, possibility and impossibility, and that modality is a matter of realis or irrealis, generally with the latter as the scope involved.Like Rescher (1968: 24ff.), Perkins believes that modality is a multi-level concept, including the following eight types:
(A）Alethic modalities, relating to the notion of truth itself;
(B）Epistemic modalities, relating to knowledge and belief;
(C）Temporal modalities, relating to time;
(D）Boulomaic modalities, relating to desire;
(E）Deontic modalities, relating to duties;
(F）Evaluative modalities, relevant to the good-bad parameter;
(G）Causal modalities, having to do with the cause-effect relationship;
(H）Likelihood modalities, bearing relationships with judgment or prediction.
According to Perkins (ibid), MVs are just one of the devices that convey various modalities, the others being lexical and grammatical ones such as MAs, modal lexical verbs, modal nouns, tense, IF-clauses and questions.Von Wright (1984）considers modality to be a matter of logic and truth conditions.In other words, modality is related to the if-then formula, inclusion and exclusion equations, and reality and hypotheticality.MVs are a type of linguistic device to convey the modality of logic and truth conditions.
In short, these various views seem to endorse that modality may cover subjectivity, irrealis, logic and truth conditions, and can be realized by various linguistic devices at both lexical and syntactic levels, including MVs.
Second, there is much disagreement on the form-notion correspondence of modality.Pedagogic grammar (e.g.Hall, 1974）holds that there is a one-to-one correspondence between modal form and function.MVs, for instance, constitute a system of static and single-level form-notion agreement.Must is regarded as compulsory and used in strong demands and commands; could is a weak modal form denoting a low responsibility or tentative request.As can be seen, the degree of modal force clearly varies from MV to MV.In contrast, most linguists oppose to prescribing such a static relationship between modal form and notion.Leech (1983）associates modal value with pragmatic function.According to pragmatics, MVs can function differently on different settings.The MVs that express imposition and impoliteness may denote courtesy and politeness on other occasions, and vice versa.Consider the following examples:
(13）Could you please finish the term paper before July 7th, the deadline?
(14）You must try some of these apples.They are very nice.
Could in Example (13）does not mean politeness or help to convey a polite request.Instead, the statement should be regarded as a command given the teacher-student relationship and the academic setting.Similarly, must in Example (14）does not indicate a sense of forcing the addressee to perform an act of reluctance but a sense of offering something beneficial to the addressee in a polite way in terms of the principles of politeness.Thus, the modal senses of MVs can vary from situation to situation.Accordingly, one modal sense can be expressed by more than one syntactic form, and one syntactic form can express more than one modal sense.
Third, there are a variety of views on the roles of modality.According to cognitive linguists (Talmy, 1988; Taylor, 1989; Langacker, 1991; Langacker, 1999), modality is the conceptualization in human mind.Realis and irrealis form the focus of the conceptualization, under which there are immediate reality, known reality, unknown reality, known irrealis and unknown irrealis along the scale of reality-potential or force dynamics.Pragmatics studies modality from the angle of speech acts.For instance, Verschueren (1999: 191-192）endorses the view that adverbs, especially those combined with MVs, can be the means of evidentiality or eliciting sources for the views concerned.Some semanticists treat modality as the quantification of opinions using logic formula.In SFL, modality is deemed as part of the interpersonal function.Thus, some modal devices may help the addresser(s）to convey information (modalization）―probability and usuality to the addressee(s）while other ones denote goods and services (modulation）―obligation and inclination.
The arguments over modality indicate that it is reasonable for linguists to adopt a variety of approaches to modality.Hence, there are five approaches to modality throughout the history of modality studies, i.e., truth-condition correspondence, the approach of cognitive linguistics, the approach of pragmatics, functional representation, and SFL.
2.1.2 Truth-condition correspondence
There are three methods under the truth-condition correspondence approach.They are the monosemantic method, the logic and truth method, and the form-function equation method.This approach proposes that modality be studied by form-function relationships such as the inference of modal senses and the form-function equation.The three methods adopt a static view on modality.
2.1.2.1 The monosemantic method
The monosemantic method to modality is taken by Traditional Grammar.Hall (1974: 195-200）claims that modality should be given a definition with the forms corresponding to the meanings or usages concerned.For instance, the following statements from Hall’s works expound his view:
(A）can expresses permission, possibility or ability, related to the present or future; could used in the past tense can indicate the euphemistic and polite sense;
(B）will is used for the future tense as well as modality, indicating willingness or volition, related to the present or future; would used in the past tense can denote the euphemistic and polite sense;