Halliday & Matthiessen (1999: 184):
Grammatically speaking, (simple) participants are realised by nominal groups, which are made up of both things and qualities. In terms of the structure of the nominal group, the cut-off point between things and qualities is between the Classifier and the Thing. Semantically, Classifiers are qualities of the ‘class’ type: they are like things and may be derived from things, but unlike things they do not have independent existence — they cannot be established in referential space and re-identified in running discourse. So for example although a ‘passenger’ is undoubtably a thing, in a passenger train, where passenger functions as Classifier, it is being construed as a quality; hence it cannot be picked up by anaphoric reference […] Grammatically, Classifiers are realised by ‘substantives’ or by ‘adjectives’, and this indeterminacy in grammatical class is symbolic of their status as qualities which are like things.