The paper presents and discusses some properties of a set of three modal particles in Modern Greek (as, na and tha) which have been claimed to form a well-defined category on the basis of a number of formal and functional characteristics (Tsangalidis 2000; cf. Holton et al. 1997), despite their apparent differences at various levels.

The three particles are mutually exclusive and what they have in common is their ability to express a modal meaning (displaying the usual alternations between epistemic and non-epistemic readings; cf. Heine 1993). More specifically, as is mainly a marker of agent-oriented modality (in the sense of Bybee et al. 1994) and can only appear in main clauses (cf. Karantzola 1993); na is the most common marker of both epistemic and agent-oriented modality as well as a general subordination marker which seems to lack any modal meaning; it can appear in either main or subordinate clauses (cf. Philippaki-Warburton 1994); moreover, a homophonous form also exists in the language which has been characterised as an "exophoric deictic" item with a specific discourse function –and its precise relation to the remaining uses of na has been extensively discussed (e.g. Christidis 1985, 1990); finally, tha may also appear in any clause as the usual marker of future-time reference and at the same time displays a great range of modal and aspectual uses (cf. Tsangalidis 1999).

This mixed class can hardly be seen as a uniform category unless either (a) any deviations from the common properties are excluded from the group on the assumption of multiple homonymies; i.e. if any non-modal uses of each particle are considered more or less accidental homonyms of the corresponding modal particle or (b) the history of the three items can support a polysemy view of all their contemporary semantic variants on the usual assumptions within grammaticalisation theory (Bybee & Dahl 1989, Bybee et al. 1994).

It is argued that the assumption of homonymy is unacceptable on theoretical, historical and empirical grounds and that therefore the relation between the various uses of each one of the particles is to be characterised as polysemy. This then has important implications for the theoretical status of the particle group, since they do not form a strictly defined category (for example, not all members of the category can be C elements in Chomskyan terms).

Thus, on one hand, in a semantics of grammar context, polysemy can only mean multiple category membership; e.g. each of the three particles appears to implicate at least two of the following categories: tense, mood, modality or discourse markers. On the other hand, the syntactic behaviour of particles cannot be expected to conform to our more general views of either the usual inflectional categories or the major lexical classes.