Thucydides, an Athenian, wrote the history of the war between the Peloponnesians and the Athenians, beginning at the moment that it broke out, and believing that it would be a great war, and more worthy of relation than any that had preceded it.
(1.1)§
And with reference to the narrative of events, far from permitting myself to derive it from the first source that came to hand, I did not even trust my own impressions, but it rests partly on what I saw myself, partly on what others saw for me, the accuracy of the report being always tried by the most severe and detailed tests possible. [3] My conclusions have cost me some labour from the want of coincidence between accounts of the same occurrences by different eye-witnesses, arising sometimes from imperfect memory, sometimes from undue partiality for one side or the other. [4] The absence of romance in my history will, I fear, detract somewhat from its interest; but if it be judged useful by those inquirers who desire an exact knowledge of the past as an aid to the interpretation of the future, which in the course of human things must resemble if it does not reflect it, I shall be content. In fine, I have written my work, not as an essay which is to win the applause of the moment, but as a possession for all time. (1.22.4)
On the plague in Athens:
All speculation as to its origin and its causes, if causes can be found adequate to produce so great a disturbance, I leave to other writers, whether lay or professional; for myself, I shall simply set down its nature, and explain the symptoms by which perhaps it may be recognized by the student, if it should ever break out again. This I can the better do, as I had the disease myself, and watched its operation in the case of others. (2.48.3)
On revolution (during the account of Corcyra)
Revolution thus ran its course from city to city, and the places which it arrived at last, from having heard what had been done before carried to a still greater excess the refinement of their inventions, as manifested in the cunning of their enterprises and the atrocity of their reprisals. Words had to change their ordinary meaning and to take that which was now given them. Reckless audacity came to be considered the courage of a loyal ally; prudent hesitation, specious cowardice; moderation was held to be a cloak for unmanliness; ability to see all sides of a question inaptness to act on any.
Frantic violence, became the attribute of manliness; cautious plotting, a justifiable means of self-defence. The advocate of extreme measures was always trustworthy; his opponent a man to be suspected. To succeed in a plot was to have a shrewd head, to divine a plot a still shrewder; but to try to provide against having to do either was to break up your party and to be afraid of your adversaries. In fine, to forestall an intending criminal, or to suggest the idea of a crime where it was wanting, was equally commended.
(3.82.3 ff)
This is the display of the inquiry ( historie ) of Herodotus of Halicarnassus, so that things done by man not be forgotten in time, and that great and marvelous deeds, some displayed by the Hellenes, some by the barbarians, not lose their glory, including among others what was the cause of their waging war on each other.
(Histories, 1.1)

In the opening scene of Cicero's Laws (1.5), Cicero, his brother Quintus and Atticus were discussing the merits of Cicero's poem on Marius, and Atticus raised the question of accuracy. Cicero demurred; accuracy, he suggested,was the business of the historian, not the poet.
"I understand, brother," said Quintus, "that you think one set of rules should be observed in history and another in poetry." "Yes," agreed Cicero, "for in history everything is meant to lead to the truth, but in poetry a great deal is intended for pleasure - although in Herodotus, the father of history , and in Theopompus, there are a countless number of legends."
Whether in tracing the history of the Roman people, from the foundation of the city, I shall employ myself to a useful purpose I am neither very certain, nor, if I were, dare I say: inasmuch as I observe, that it is both an old and hackneyed practice, later authors always supposing that they will either adduce something more authentic in the facts, or, that they will excel the less polished ancients in their style of writing. Be that as it may, it will, at all events, be a satisfaction to me, that I too have contributed my share to perpetuate the achievements of a people, the lords of the world; ...
The traditions which have come down to us of what happened before the building of the city, or before its building was contemplated, as being suitable rather to the fictions of poetry than to the genuine records of history, I have no intention either to affirm or refute. ...
I would have every man apply his mind seriously to consider these points, viz. what their life and what their manners were; through what men and by what measures, both in peace and in war, their empire was acquired and extended; then, as discipline gradually declined, let him follow in his thoughts their morals, at first as slightly giving way, anon how they sunk more and more, then began to fall headlong, until he reaches the present times, when we can neither endure our vices, nor their remedies.
This it is which is particularly salutary and profitable in the study of history, that you behold instances of every variety of conduct displayed on a conspicuous monument; that from thence you may select for yourself and for your country that which you may imitate; thence note what is shameful in the undertaking, and shameful in the result, which you may avoid. But either a fond partiality for the task I have undertaken deceives me, or there never was any state either greater, or more moral, or richer in good examples, nor one into which luxury and avarice made their entrance so late, and where poverty and frugality were so much and so long honoured; so that the less wealth there was, the less desire was there. Of late, riches have introduced avarice, and excessive pleasures a longing for them, amidst luxury and a passion for ruining ourselves and destroying every thing else. But let complaints, which will not be agreeable even then, when perhaps they will be also necessary, be kept aloof at least from the first stage of commencing so great a work. We should rather, if it was usual with us (historians) as it is with poets, begin with good omens, vows and prayers to the gods and goddesses to vouchsafe good success to our efforts in so arduous an undertaking.