A practice, not a theory
The ancient discussion of how history should be written is mostly conducted through the historians' own work rather than in freestanding methodological treatises. Thucydides sets out his working principles in the opening of the Peloponnesian War — the priority of documents and eyewitness over received tradition, the distinction between aitiai (underlying causes) and prophasis (proximate occasion), the refusal to dress speeches in language the speakers would not have used. Polybius takes the practice over in Book XII of the Histories and gives it a name: pragmatikē historia — pragmatic history, the kind of historical writing that can serve practical political and military education. The Latin historians (Sallust, Livy, Tacitus) inherit the Greek discussion and adapt it to Roman conditions.
The practice that emerges is not modern academic history. It is the older inquiry into how the writer of history can write responsibly — what the writer owes the events, what the writer owes the reader, and what kind of evidence is admissible to the work.
The three Polybian principles
Polybius's articulation of the method is the most explicit ancient statement. Pragmatic history, he argues, requires three things.
First, the historian must work from documents and from personal investigation — interviewing participants, visiting battlefields, reading the inscriptions — rather than from earlier narratives. History that is compiled from other histories accumulates the errors without correcting them.
Second, the historian must have practical political and military experience. An armchair writer cannot read the conditions that produced the events; he will mistake what an honest description requires; he will pretend that the strategic and political decisions he has not lived in were simpler than they were. Polybius reserves some of his sharpest language for historians whose lack of practical experience produces what he calls armchair history (ek tou kathēmenou).
Third, the historian must seek causes — aitiai — rather than rest with proximate occasions or chronological description. The historian who cannot explain why an event occurred has not yet done the historian's work.
These three principles describe a working ideal. Few ancient historians met all three in every passage. The principles set the standard the surviving major histories were either trying to meet or visibly departing from for reasons of their own.
The Roman elaboration
The Roman historians inherit the Polybian framework and adapt it. Sallust takes the cause-seeking requirement seriously: the proem to the Catilina is an explicit move from the proximate occasion (the conspiracy) to the underlying causes (the moral condition of the polity after 146 BCE). Livy works in a different register — annalistic, narrative, drawing the moral argument out of the cumulative weight of exempla rather than through analytical digression — but his preface is consistent with the Polybian demand that history serve civic education. Tacitus is the most Polybian of the three in temperament: the psychological-political analysis in the opening of the Annales is causal explanation pressed to its sharpest form.
The Roman elaboration also adds something the Greek tradition had mostly missed. Roman historians, working in a culture that took mos maiorum — ancestral custom — as part of the constitutional order, treat the historian's responsibility as partly civic. The work is for the citizen body; it serves the polity by making its memory of itself more honest; it has, in this sense, a political function the Thucydidean tradition did not insist on quite so explicitly.
What the practice required
The ancient idea of historical method depended on conditions later academic history has not always shared. The historian was assumed to be a man of practical political experience writing for other such men. The audience was small and educated; the standards of evidence were the standards of the political class. The work was published, in the ancient sense — read aloud at gatherings, copied by professional scribes, traded between literate households — and the reader was expected to bring his own knowledge of the events to the reading.
These conditions are not modern. What survives across the gap is the idea that history is a practice with intellectual standards of its own — that the historian owes documentation, causal explanation, and the analytical seriousness that makes the work usable to thinking readers — and that the practice has a civic function distinct from chronicle, antiquarianism, or entertainment.
Why the platform reads it
The platform reads the ancient inquiry into historical method because it stands behind every primary text the corpus carries from the Roman historiographical tradition. To read Livy as a serious historical writer rather than as a moralist with a narrative gloss; to read Sallust as a working diagnostician rather than as a stylist; to read Tacitus as a clinical analyst rather than as a senatorial opponent of empire — each requires understanding what these writers took the historical method to be. The standards are not the same as those of modern academic history. They are the older standards out of which modern historical practice was eventually shaped.