Modality is a semantic category in which speakers express their attitude and commitment toward communicated propositions.  It belongs to the interpersonal metafunction.  Modality can be deontic or epistemic and is most obviously manifested in modal verbs.  In deontic modality speakers express obligation and permission.  In epistemic modality speakers express degrees of certainty.  The ideological significance of modality is that it ‘suggests the presence of an individual subjectivity behind the printed text, who is qualified with the knowledge required to pass judgement' (1991: 64).  Modality is therefore 'an important part of the practices by means of which claims to authority are articulated and legitimated authority is expressed' (Fowler 1985: 73).