Halliday & Matthiessen (2004: 207):
Mental processes … are not kinds of doing, and cannot be probed or substituted by do.
When the clause refers to present time, the tense of the verbal group serving as Process is the simple present rather than the present–in–present that is characteristic of ‘material’ clauses.
In a ‘mental’ clause, the unmarked present tense is the simple present … But in a ‘material’ clause the unmarked present tense is the present in present …
In contrast to the Subject, the Complement is realised by a nominal group that can denote entities of any kind …
This process of sensing may be construed either as flowing from a person’s consciousness [‘like’ type] or as impinging on it [‘please’ type]; but it is not construed as a material act.
While ‘material’ clauses are concerned with our experience of the material world, ‘mental’ clauses are concerned with our experience of the world of our own consciousness. They are clauses of sensing: a ‘mental’ clause construes a quantum of change in the flow of events taking place in our own consciousness.