The ancient Greek historian Herodotus wrote in the mid fifth-century BC about the centuries-long enmity between the Greeks and the Persians, which climaxed in the Persian Wars. He had to rely primarily on oral accounts of events that occurred centuries earlier, and consequently it was difficult for him to be accurate about those events. He deserves to be called the "father of history" for his achievement in committing oral history to writing.
What is his method? When stories conflict, he often gives us both sides, refusing to select which is right. For example at Book 1 section 5 he says : "These are the stories of the Persians and the Phoenicians. For my own part, I will not say that this or that story is true." He goes on to take a stand about a particular fact, but his focus remains primarily on what people said and thought about history. In many ways he remains a storyteller.
He also interprets events in terms of his own time. His discussion of the establishment of the Persian monarchy (3.80 f.) is recounted in terms of a fifth-century Athenian political debate. Taken literally, the story would mean that the Persians discovered democracy and oligarchy in the 560's BC, rejected them in favor of monarchy, and selected their monarch by a contest of neighing horses.
In contrast, Thucydides had a different project, and a different method. He wrote mostly about contemporary events which he could witness directly: the Peloponnesian War between Athens and Sparta. He writes of distant events only when they are necessary background to his own topic.
But he is also armed intellectually, with a historical method that is a qualitative improvement upon that of Herodotus. Why do men need a method in understanding history? He tells us at Book 1 section 20: "For men accept from one another hearsay reports of former events, neglecting to test them . . ." He follows this with a specific example of a historical error accepted by the Athenians of his own day, then explains this error: "So averse to taking pains are most men in the search for the truth, and so prone are they to turn to what lies ready at hand."
What is his method? Thucydides distinguishes himself from storytellers: "But as to the facts of the events of the war, I have thought it right to give them not as ascertained by any chance informant nor as seemed to me probable, but only after investigating with the greatest possible accuracy each detail . . . possibly the absence of the fabulous from my narrative will seem less pleasing to the ear. But if those who wish to have a clear view both of the events that have happened and those which will likely occur again adjudge my history profitable, this will be enough for me. And indeed it has been composed not as an essay for the present moment, but as a prize possession for all time." (1.22)
The essence of the storyteller's project is to create a narrative that is pleasing to the reader. The standard of what is good is what is appropriate for a given audience. The essence of the historians method, in contrast, is to focus on the facts. The storyteller subordinates the facts to the pleasingness of the story. The historian subordinates his narrative to the accuracy of the facts.
Thucydides is not, however, satisfied with simply cataloguing the particular events of the war. He wants to know the reasons for the conflict. What men said were the causes of the war was a series of minor events, mere pretenses which he proceeds to relate. But the truest cause was in the growth of Athenian power, and the fear it brought to the Spartans. This is an integration of those events into a broad abstract reason for the war.
But his understanding of the causes of war goes even deeper, to a moral level. This is epitomized in the so-called Melian dialogue (5.84f.). This is a grand-scale integration of the deepest reasons for war, which demonstrates why men need a morality based on reason rather than force if war and slavery are to be avoided. These are his ultimate reasons why Athenian power grew as it did, why the allies of the Spartans were afraid, why their allies demanded that the Spartans fight, and why the fight had to be to the death. It is because the Athenians rejected reason in dealing with other men.
What Thucydides did was to state explicitly that human history is causal, and that causes can be proximate and long-range. Events are likely to repeat if the same causes occur again. Understanding long-range causes is a guide to the future as well as the past.
Thucydides presents more than an application of causation to history--his work may hold the very discovery of causation as a principle of human affairs. This is an astounding achievement. It makes history, which to him meant any investigation into the facts of human affairs, scientific.