[...] interpersonal metaphor [is] based on the semantic relationship of projection. In this type the speaker’s opinion regarding the probability that his observation is valid is coded not as a modal element within the clause, which would be its congruent realization, but as a separate, projecting clause in a hypotactic clause complex. To the congruent form 'it probably is so' corresponds the metaphorical variant 'I think it is so', with 'I think' as the primary or ʻalphaʼ clause.
- Halliday (1985), Pag. 332