But his philological concerns now were only a small part of a much larger project to make sense of the nature of the Greek culture itself. His work in this period shows a theoretical sophistication far beyond his earlier efforts, extending his theory of literacy toward a theory of culture itself. He said of the Dipylon inscription, a poetic line scratched into a vase and the earliest Greek writing known at the time, "Here in this casual act by an unknown hand there is announced a revolution which was destined to change the nature of human culture." It is this larger point about the differences between oral and literate culture that represents Havelock's most influential contribution. Walter J. Ong, for example, in assessing the significance of non-oral communication in an oral culture, cites Havelock's observation that scientific categories, which are necessary not only for the natural sciences but also for historical and philosophical analysis, depend on writing. These ideas were sketched out in "Preface to Plato" but became central to Havelock's work from "Prologue to Greek Literacy" (1971) onward.