In the same fashion systematic history too consists of three parts, the first being the industrious study of memoirs and other documents and a comparison of their contents, the second the survey of cities, places, rivers, lakes, and in general all the peculiar features of land and sea and the distances of one place from another, and the third being the review of political events;
and just as in the case of medicine, many aspire to write history owing to the high opinion in which the science is held, but most of them bring to the task absolutely no proper qualification except recklessness, audacity, and roguery,
courting popularity like apothecaries, and always saying whatever they regard as opportune in order to curry favour for the same of getting a living by this means. About them it is not worth saying more.
Some of those again who appear to be justified in undertaking the composition of history, just like the theoretical doctors, after spending a long time in libraries and becoming deeply learned in memoirs and records, persuade themselves that they are adequately qualified for the task, seeming indeed to outsiders to contribute sufficient for the requirements of systematic history, but, in my own opinion, contributing only a part.
For it is true that looking through old memoirs is of service for knowledge of the views of the ancients and the notions people formerly had about conditions, places, nations, states, and events, and also for understanding the circumstances and chances which beset each nation in former times.
For past events make us pay particular attention to the future, that is to say if we really make thorough inquiry in each case into the past.
But to believe, as Timaeus did, that relying upon the mastery of material alone one can write well the history of subsequent events is absolutely foolish, and is much as if a man who had seen the works of ancient painters fancied himself to be a capable painter and a master of that art.
Walbank Commentary