Halliday & Matthiessen (2014: 248):
Obviously clauses construing doing and clauses construing sensing are different in meaning, but that is not enough to make them constitute distinct grammatical categories. There are indefinitely many ways of drawing lines on purely semantic grounds, for example, by invoking contextual considerations ‘from above’, as we do when we describe the semantic strategies specific to a particular situation type; but the question we are concerned with here is which of these have systematic repercussions in the grammar.