Quomodo historia conscribenda sit

After all his preparations are made he will sometimes begin without a preface, when the subject matter requires no preliminary exposition. But even then he will use a virtual preface to clarify what he is going to say.

Whenever he does use a preface, he will make two points only, not three like the orators. He will omit the appeal for a favourable hearing and give his audience what will interest and instruct them. For they will give him their attention if he shows that what he is going to say will be important, essential, personal, or useful. He will make what is to come easy to understand and quite clear, if he sets forth the causes

The best historians have written prefaces of this sort: Herodotus, writing history to preserve events from time’s decay, great and glorious as they were, telling of Greek victories and barbarian defeat; Thucydides too, with his expectation that the war would be great, more memorable, and more important than any that had gone before; and in fact the sufferings in that war were considerable.