This essay investigates the assumptions which underlie Dionysius’ critical practice through a close examination of some of the more programmatic passages of the On Thucydides. Dionysius is concerned to determine why classical authors made the choices they did and how such knowledge can be instructive in Dionysius’ own day. In this investigation, Dionysius typically retrojects back on to the subjects of his essays his own sense of what is morally and stylistically appropriate; some of Thucydides’ faults lie in the fact that he did not share Dionysius' own classicising view of the achievements of classical Athens. The essay also discusses Dionysius’ defence against the charge that the best critics must themselves also be able to compose great works in the genres in which they claim expertise.