How did Tacitus use Agrippina the Younger’s memoirs as a historical source?

The Roman historian Tacitus utilized the personal memoirs of Agrippina the Younger as one of several firsthand sources when compiling his historical accounts, specifically within his work the Annals.1

Tacitus consulted these memoirs alongside other primary materials—such as the Senate’s records, official journals, and the military memoirs of Gnaeus Domitius Corbulo—to reconstruct the history of the Julio-Claudian dynasty.1

However, Tacitus’s approach to such sources was not purely objective. When examining earlier periods, he sought to determine the factual reliability and political biases of his authorities. His own historical method was often influenced by his strong convictions, which led him to prioritize personal factors over immediate circumstances. Consequently, while he incorporated these firsthand accounts, he maintained control over the performance of his characters, resulting in a narrative that is considered magnificent writing but not necessarily strict history.1