Halliday & Matthiessen (2004: 604-5):
Projection can also be manifested interpersonally in the form of a modal Adjunct. … Unlike the logical and experiential manifestations, the interpersonal manifestation does not represent the Sayer or Senser; rather it enacts the speaker’s opinion — an enactment of his or her degree of commitment to the proposition: the proposition is assessed as being projected by someone other than the speaker. This type of assessment is known as ‘evidentiality’: the modal Adjunct is used to indicate the evidential status of the proposition. … evidentiality is related to ‘verbal’ clauses and ‘mental’ clauses of perception.