Halliday & Matthiessen (1999: 514-5):
This part of the grammar, then — the grammar of clauses — , constitutes a theory about the types of process that make up human experience. In English (which is probably fairly typical), the three principal categories that we are calling the material, mental and relational are rather clearly distinct on a number of formal grounds; the other three appear as mixed or intermediate categories lying on the borderlines. (In fact the category of verbal process is more clearly distinct than the other two; and in view of its central place in the semantic system, we have treated it throughout the present study as a primary category.) The total picture is of a continuum; but not between two poles — rather something that we would represent in the form of a circle.