Halliday & Matthiessen (1999: 143-4):
As always in language, the picture that emerges from a consideration of a multiplicity of properties is far from simple; it is multifaceted. But it is possible to bring out certain salient features of the system of sensing as suggested in Figure 4-6.
Emotion seems to be closer to quality-ascription than to a prototypical process; it arises from, but does not create, projections. In contrast, perception is essentially closer to behavioural processes. Cognition and desideration are different from both in that they can project (i.e., bring the content of consciousness into existence), can stand for modalities, and are not in general like either behaviour or ascription; they may be interpreted as the most central classes of sensing. Cognition is arguably closer to perception than desideration is — there are certain cross-overs like see in the sense of 'understand' alongside its basic sense of visual perception, and both can be construed in an active mode as processes of behaviour.