Halliday & Matthiessen (1999: 198-9):
This resource, the construal of systematically related lexico-semantic sets, illustrates well the principle of "lexis as most delicate grammar" (Halliday, 1961; Hasan, 1987; Matthiessen, 1991b; Cross, 1993). We have discussed above the principle that categories in the experiential grammar are ordered in delicacy, so that starting from the very general types of process that are construed into figures, we can differentiate both processes and participants into finer and finer subcategories, until we reach the degree of differentiation that is associated with the choice of words (lexical items). Note that it is not (usually) the lexical items themselves that figure as terms of the systems in the network. Rather, the systems are systems of features, and the lexical items come in as the synthetic realisation of particular feature combinations. Thus lexis (vocabulary) is part of a unified lexicogrammar; there is no need to postulate a separate "lexicon" as a pre-existing entity on which the grammar is made to operate.