Halliday (2008: 180-1):
But, if we see grammaticalisation as the dimensionalising of semantic space, are there other comparable processes? Lexicalisation is the complementary process, likewise in perspective “from above”, so not a shift from the grammatical to the lexical but a construal of meaning in lexical terms — that is, those regions in the semantics of a language that are realised as (sets of) lexical items.