1. The paraphrase relation or the cognitive synonymy between sentences has played a key rôle in the development of the theory of transformational grammar. It was a remarkable feat of early transformational grammarians to discover that the cognitive synonymy as well as the similarity of certain selectional restrictions between pairs of active and passive sentences can be accounted for by positing single deep structures, and by relating the two types of surface sentences by transformations: I have in mind the restrictions which apply to sentences such as John frightens sincerity, Sincerity is frightened by John.