Halliday & Matthiessen (1999: 466):
A characteristic of work on grammatical semantics, where this has been based on linguistics or on natural language philosophy, is to move in at the lower ranks of the grammar rather than the higher ones, and to start with classes rather than with functions. This is a continuation of the method of traditional grammar, which (because it originated with the study of observable features of language) was word-oriented and leant heavily on word classes in its descriptive statements. We find this tendency in discussions of word classes and their semantic values — the issue of the proper interpretation of adjectives, the exploration of various verb types, and so on.