Thucydides' History is extraordinarily dense and complex. His particular ancient Greek prose is also very challenging, grammatically, syntactically, and semantically. This has resulted in much scholarly disagreement on a cluster of issues of interpretation; as is common among much ancient Greek language.
Strata of composition
It is commonly thought that Thucydides died while still working on the History, since it ends in mid-sentence and only goes up to 410 BC, leaving six years of war uncovered. Furthermore, there is a great deal of uncertainty whether he intended to revise the sections he had already written. Since there appear to be some contradictions between certain passages in the History, it has been proposed that the conflicting passages were written at different times and that Thucydides' opinion on the conflicting matter had changed. Those who argue that the History can be divided into various levels of composition are usually called "analysts" and those who argue that the passages must be made to reconcile with one another are called "unitarians". This conflict is called the "strata of composition" debate.
The History is notoriously reticent about its sources. Thucydides almost never names his informants and alludes to competing versions of events only a handful of times. This is in marked contrast to Herodotus, who frequently mentions multiple versions of his stories and allows the reader to decide which is true. Instead, Thucydides strives to create the impression of a seamless and irrefutable narrative. Nevertheless, scholars have sought to detect the sources behind the various sections of the History. For example, the narrative after Thucydides' exile (4.108ff.) seems to focus on Peloponnesian events more than the first four books, leading to the conclusion that he had greater access to Peloponnesian sources at that time.
Frequently, Thucydides appears to assert knowledge of the thoughts of individuals at key moments in the narrative. Scholars have asserted that these moments are evidence that he interviewed these individuals after the fact. However, the evidence of the Sicilian Expedition argues against this, since Thucydides discusses the thoughts of the generals who died there and whom he would have had no chance to interview. Instead it seems likely that, as with the speeches, Thucydides is looser than previously thought in inferring the thoughts, feelings, and motives of principal characters in his History from their actions, as well as his own sense of what would be appropriate or likely in such a situation.