Halliday & Matthiessen (2014: 184):
One aspect of the highly grammaticalised nature of modality in English is – not surprisingly – that it has expanded its domain of realisation: within the clause, this domain includes not only Finite verbal operators (e.g. will) but also Adjuncts within the Mood element (e.g. probably); and beyond the clause, it includes ‘bi-clausal’ realisations such as I think that ...; and it is probable that serving as ‘explicit’ manifestations of ‘subjective’ and ‘objective’ orientation …. Such manifestations are, in fact, metaphorical extensions of the system of modality … . Since they are metaphorical realisations, they are also analysed as if they were expressions serving as mood Adjuncts