Why did Thucydides distrust eyewitness accounts without careful checking?

Thucydides distrusted eyewitness accounts because he recognized that human perception is inherently fallible. He understood that eyewitnesses often provided unreliable information due to faulty memory or personal bias, even regarding events he had observed himself.1

To ensure the accuracy of his history, Thucydides conducted as thorough an inquiry as possible rather than relying solely on direct observation or unverified reports. His commitment to truth was driven by a desire to provide a reliable record for posterity, focusing on the human nature and political motives behind the events of the Peloponnesian War rather than mere storytelling. By meticulously checking his sources and maintaining a strict chronological scheme, he sought to create a work that would remain useful to future students of history.12