Halliday & Matthiessen (2014: 250):
While the Senser is construed as being endowed with consciousness in ‘mental’ clauses, there is no trace of this pattern in ‘material’ clauses. In ‘material’ clauses, no participant is required to be human, and the distinction between conscious and non-conscious beings simply plays no part. The Actor of a ‘material’ clause is thus much less constrained than the Senser of a ‘mental’ clause.