Halliday & Matthiessen (1999: 143):
Finally, when the different types of sensing are construed metaphorically as things, they are reified in different ways. Perception, cognition and desideration are reified as bounded, i.e. countable things, such as sight(s), thought(s), plan(s), whereas emotions are reified as unbounded things, i.e. masses, such as anger, fear, frustration.⁵ That is, emotion is construed as boundless — like physical resources such as water, air, iron and oil (cf. Halliday, 1990). Indeed, one can see from Lakoff & Kovecses's (1987) discussion of the cognitive model of anger in American English that a number of the metaphors for anger construe it as concrete mass (e.g. as a fluid contained in the body: He was filled with anger, She couldn't contain her joy, She was brimming with rage). In being construed as unbounded mass, emotions are again more like qualities (cf. the unbounded strength, height, heaviness, redness).
⁵ A few processes of cognition are also unbounded, e.g. knowledge, realisation, understanding.