Halliday & Matthiessen (1999: 615, 616):
General terms are not necessarily abstract; a bird is no more abstract than a pigeon. But some words have referents that are purely abstract — words like cost and clue and habit and tend and strange; they are construing some aspect of our experience, but there is no concrete thing or process with which they can be identified. … experience is being reconstrued in order to build up a form of knowledge that is systematically organised and explicit.