What is the historian supposed to do? How does his task relate to that of other kinds of writer?
This is a question that exercised ancient historians and others who theorized about history in antiquity. Cicero was one of those who did the latter, and in On the Laws 1.4-5 he gives a brief summary of his view.
The question of the task of the historian arises from a discussion about the rules that govern different literary genres, for “every genre is a mode of communication, with its own vocabulary and conventions.” 1 On the Laws begins with a question from one of the characters about an oak tree in Arpinum mentioned in Cicero’s lost poem Marius at which Marius saw a divine omen. Atticus wonders if the tree that he, Marcus [Cicero], and Marcus’ brother Quintus have just come upon is the selfsame tree.
Marcus first warns against inquiring too closely into tradition–things that are passed on in legend and myth can’t bear that kind of scrutiny. Atticus isn’t quite satisfied; he wants to know whether this and other items in the Marius are fact or fiction. As he says, “some people expect you to tell the truth.” 2
Marcus’ reply is interesting. He notes, “I certainly don’t want to be thought a liar.” As he goes on to assert, however, this simple dichotomy of truth and falsehood is the wrong one for a work of imaginative literature, though those of a philosophical cast of mind–like Plato, for instance, and Augustine–often were wont to conflate the fiction of poetry with mendacia, “lies.” The poet has his own kind of “truth,” perhaps; in any case, it would be wrong to stand in front of him and yell, “LIAR!!!” Cicero says:
[T]he people you mention are being naive; they are demanding in this case the kind of truth expected of a witness rather than a poet.
Quintus realizes that Marcus is making a point about literary genre and replies accordingly: “I take it, Marcus, that in your view one set of rules must be followed in a work of history, another in a poem.” “Yes,” Marcus says, “because in the former everything is measured by the standard of truth, Quintus, whereas in the latter the main purpose is to entertain.” 3 While there is a kind of truth expected of a poet, it is not the same as that expected of a historian. The historian is to be a witness to the past. He must testify.
The way in which Cicero views history makes it a close cousin of oratory for him. Hence the remark of Atticus that follows shortly, in which he urges Marcus to take up the task of writing historical literature:
For, as I myself recognize and have often heard you say, our literature is lacking in the field of history. You above all people could supply this need, since, as you often maintain, this kind of writing is so closely akin to oratory.
As Niall Rudd notes on this passage,
[S]everal pupils of the orator Isocrates (436-338) became historians, e.g. Ephorus and Theopompus. Cicero held that history should supply instances (exempla) which could be used by orators, and that rhetoric (especially the rhetoric of display) could show historians how to compose vivid descriptions.
Rudd refers to two further passages in Cicero for corroboration. One is On the Orator 2.62-4:
But I return to my subject. Do you see how far the study of history is the business of the orator? I know not whether it is not his most important business, for flow and variety of diction; yet I do not find it anywhere treated separately under the rules of the rhetoricians. Indeed, all rules respecting it are obvious to common view; for who is ignorant that it is the first law in writing history, that the historian must not dare to tell any falsehood, and the next, that he must be bold enough to tell the whole truth? Also, that there must be no suspicion of partiality in his writings, or of personal animosity?  These fundamental rules are doubtless universally known. The superstructure depends on facts and style. The course of facts requires attention to order of time, and descriptions of countries; and since, in great affairs, and such as are worthy of remembrance, first the designs, then the actions, and afterwards the results, are expected, it demands also that it should be shown, in regard to the designs, what the writer approves, and that it should be told, in regard to the actions, not only what was done or said, but in what manner; and when the result is stated, that all the causes contributing to it should be set forth, whether arising from accident, wisdom, or temerity; and of the characters concerned, not only their acts, but, at least of those eminent in reputation and dignity, the life and manners of each.  The sort of language and character of style to be observed must be regular and continuous, flowing with a kind of equable smoothness, without the roughness of judicial pleadings, and the sharp-pointed sentences used at the bar. Concerning all these numerous and important points, there are no rules, do you observe, to be found in the treatises of the rhetoricians.
History is to be held to the standard of the truth, and because it tells the truth it has a didactic function; it teaches. But, for Cicero, “to teach” doesn’t just mean “to say what happened in the past; to give information.” History is also to connect readers to that past and to provide exempla for the orator. In doing so, the writer of history, for Cicero, does not just explain causation, but is permitted (or, really, required) to evaluate what has occurred–he is to register his “approval”–and to deal with character as well. To really understand a figure, we need to know not just his “acts,” but also his manner of life, with all done in a pleasing style.
We need to know these things in order to connect ourselves with our own past. If we do not, we remain children with respect to our awareness of our own situation and historical moment, however large our tally of years may wax. Cicero writes in The Orator:
He must be acquainted also with the history of past ages and the chronology of old time, especially, indeed, as far as our own state is concerned; but also he must know the history of despotic governments and of illustrious monarchs; and that toil is made easier for us by the labours of our friend Atticus, who has preserved and made known the history of former times in such a way as to pass over nothing worth knowing, and yet to comprise the annals of seven hundred years in one book. For not to know what happened before one was born, is to be a boy all one’s life. For what is the life of a man unless by a recollection of bygone transactions it is united to the times of his predecessors? But the mention of antiquity and the citation of examples give authority and credit to a speech, combined with the greatest pleasure to the hearers.
For Cicero, historiography is part of the general field of literature, but it occupies its own terrain with its own relation to particular kinds of truth: truths that have an impact on the development of the person as an individual with a history, in a history.