Halliday & Matthiessen (2014: 74):
Word classes were traditionally called ‘parts of speech’, through mistranslation of the Greek term meroi logou, which actually meant ‘parts of a sentence’. These began, with the Sophists, as functional concepts, rather close to Theme and Rheme; but they were progressively elaborated into, and replaced by, a scheme of word classes, defined by the kinds of inflection that different words underwent in Greek … .