Halliday & Matthiessen (1999: 186):
Qualities tend be experientially simple, specifying values along a single dimension or scale such as size, weight, loudness, colour, according to either scalar or binary distinctions (e.g. scalar: ‘large’ — ‘small’, ‘tall’ — ‘short’; binary: ‘male’ — ‘female’, ‘dead’ — ‘alive’). Things, on the other hand, tend be experientially more complex than qualities. They are often definable in terms of an elaborate taxonomy where several dimensions (parameters) are needed to distinguish them.