Halliday & Matthiessen (1999: 206):
These values may be of three kinds, according to the type of contrast they set up:(i) binary, e.g. ('dead'/'alive');(ii) scalar, e.g. ('happy/'sad');(iii) taxonomic, e.g. 'wooden'/'plastic'/'stone'/... .Of the three, the most complex, experientially, are the taxonomic qualities; for example, the taxonomy of materials (whether a folk taxonomy or one that is more scientifically informed) is based on a variety of different features, such as (in a folk taxonomy) its appearance, its texture, its range of functions, its relative value in different contexts, and the like. Taxonomic qualities are thus the closest to things; they are often realised as denomina! adjectives, or even as nouns, and they tend to function as Classifier rather than Epithet (i.e. they sort things into classes rather than describing them).