Halliday & Matthiessen (1999: 143):
… 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, ie countable things, such as sight(s), thought(s), plan(s), whereas emotions are reified as unbounded things, ie masses, such as anger, fear, frustration. That is, emotion is construed as boundless — like physical resources such as water, air, iron and oil. … In being construed as unbounded mass, emotions are again more like qualities (cf unbounded strength, height, heaviness, redness).