It is hardly necessary to recapitulate Rhys Roberts' cumulative and convincing proof that the treatise ‘On the Sublime’ was not written by Cassius Longinus, the tutor of Zenobia, but belongs to the early days of the Empire. Not the least convincing of the arguments for this date is the fact that the treatise is suggested by and put out as a substitute for the Περ ״ϒψоνς of Caecilius of Calacte, who according to Suidas taught rhetoric (σоφστενσε) in Rome in the time of Augustus. Now Caecilius was an intimate friend of Dionysius of Halicarnassus (Ep. ad Pomp. 776 τῷφιλττῳ Kαικιλψ): they were both Atticists and fellow-workers in leading literary Romans back to the best models of Greek prose style. But Dionysius is no candidate for the authorship of the extant treatise, which is not one that he could have written. On the other hand he gives a plain indication by which to identify its writer, which Rhys Roberts mentioned but did not adopt. It is the object of this paper to put this identification seriously forward.