The subject of this contrastive study is fourfold: (a) it takes into account the inference of languages in contact in the diachronic development of English language; and it builds up a hypothesis on the origin of periphrastic DO; (b) this study lays its foundations on established facts from diachrony and etymology to claim that the grammaticalization of DO did not entail any process of desemantisation; (c) it also resorts to an invariant-meaning approach to show that the auxiliary DO – just like its lexical counterpart – is not meaningless in natural language; (d) it carries out a critical analysis of current trends on either a binary categorization or a ternary categorization for DO-forms (lexical verb (vs. proverb) vs. auxiliary). Then, this study shows that the proform DO SO can indeed substitute for purely stative predicates; and, as an operator of ‘thesis’, DO has an enunciative function which accounts for its occurrences as well as its non-occurrences in the linear structuring.