The polysemantic approach is advocated by linguists like Leech (1969， 1971）and Palmer (1974， 1979).In fact， to simply regard MVs as polysemous is not enough.Coates (1983： 11-12）proposes a Gradience Model and a Fuzzy Set to account for the indeterminacy of MVs， to the effect that MVs can be epistemic， deontic or dynamic on a continuum without a clearly-cut boundary and that MVs exhibit core， skirt and periphery from membership to non-membership in modality types.According to Coates (1983： 14-16)， there are three types of indeterminacy as related to MVs： (a）gradience； (b）ambiguity； (c）merger.Gradience refers to the situation where MVs have core and peripheral meanings (e.g.the core and peripheral meaning of can are “ability” and “possibility” respectively)， but all these modal meanings may be applicable depending on the use of subjects and the judgment of discourse participants.In other words， the modal meanings of MVs may shift from type to type depending on the contexts.Ambiguity refers to the situation where it is hard to decide which of the modal meanings is intended.Merger refers to the situation where the modal meanings of MVs are not mutually exclusive in certain contexts.This can be described as contextual neutralization.
For instance， can indicates both dynamic (about ability）and deontic (about permission）meanings in many cases.However， when MVs are combined with MSAs， such ambiguities can be dissolved.Consider the following：
(10）This language shall thereafter be the language of the proceedings for all notifying parties.
(11）Essentially this means that the joint venture must operate on a market， performing the functions normally carried out by other undertakings operating on the same market.
The MSA thereafter in Example (10）serves as a kind of evidential source， i.e.it provides the justification for the order or command.In comparison， the MSAs essentially and normally in Example (11）indicate the principle for the order or command， i.e.it is necessary that the decree be obeyed unless exceptional events occur.In this way， shall will not be regarded as indicating a personal volition， and must will not be treated as an imposition of a certain person’s will， either.This dissertation assumes that given the indeterminacy， gradience and fuzziness of MVs in discourse， MSAs can contribute to reducing or eliminating the ambiguity concerned.
Second， it considers that the interpersonal metafunction of MSAs is related to linguistic evolution， pragmatics and cognition.
Linguists who are engaged in diachronic studies and structuralism have explored the truth conditions and evolution of modal devices.Lyons (1977）puts forward the concept of modal harmony.For instance， will easily， could always， and must at least frequently occur， but *may certainly， *could surely， *must possibly and *must maybe (* indicates non-acceptability）seldom occur.These studies are conducive for people to understand how MSAs occur， and what roles MSAs show in combining with MVs as a means of modal synergy.
From pragmatic and cognitive perspectives， Levinson (1983)， Lyons (1995)， Lappin (1996)， and Verschueren (1999）study MVs and MAs in terms of scalar quantifications， speech acts and adaptation.
From the point of view of emergent grammar based on corpus studies， linguists such as Geere (2001)， Wray (2002)， Jaszcolt (2003： 657-663）and Terkourafi (2003）show an interest in exemplifying idiomaticity and collocatibility of MV + MA and other types.Consider the following：
(12）Crucially， as I hope to demonstrate， what makes press releases unique ― and what constitutes the prime object of my investigation ― is that their only raison d’être is to be retold： in particular， I shall provide evidence from the data that they are meant to be ‘continued’ as accurately as possible， preferably even verbatim， in news reporting.Indeed， it could be argued that press releases just do not exist unless they are also， in someway or another， ‘picked up’ by journalists.Significantly， while most of the business organizations that I contacted were able to supply me with meticulously ordered files of press releases that had been issued by them even years back， the journalists that agreed to collect press releases for this research project afterwards reported that they had had to constantly remind themselves of their promise to do so and， even then， dozens of the press releases I received from them showed obvious signs of crumpling： clearly， for press releases there is no middle course between a sad end in the waste-paper bin or the glory of coverage in the press.
In Example (12)， a number of cases of modality supplementing occur， e.g.crucially + hope， in particular + shall + as accurately as possible + preferably even， indeed + could be argued that + just + also + in some way or another， significantly + were able to， and had had to + constantly.In these cases， there are layers of modal devices at the clause level.These cases display a variety of patterns， such as adverb + lexical verb， adverb + MV， and adverb + verbal group.This strongly indicates that the study of MSAs as a means of modality supplementing in the pattern of MV + MSA could be based on the existing literature concerned to some extent.In general， MSAs are employed to exhibit the multiple layers or aspects of modality in discourse.
These studies are conducive for people to understand how MSAs occur， and what roles MSAs show in combining with MVs as a means of modal synergy.Based on the previous research on modality and SFL， this dissertation holds that MSAs should be studied qualitatively in the following way：
A.MSAs as a modality system are a matter of choices， in which modal potentials are displayed in relation to interpersonal functions depending on contexts， addresser - addressee relationships， genres and other functional factors (cf.Martin， 1997)；
