Halliday & Matthiessen (2004: 464):
In ‘verbal’ process clauses, therefore, he said that simply attests his production of the wording, whereas he said so raises the issue of whether what he said is in fact the case. With ‘mental’ process clauses the picture is more complex, since the reference form that tends to be associated with certainty and the substitute so with uncertainty; the principle is actually the same, but it is operating in a different environment. The principle is that a substitute does not refer; it simply harks back. It thus has the general semantic property of implying, and so excluding, possible alternatives; cf the nominal substitute one as in a big one, meaning ‘there are also small ones, and I don’t mean those’.