Edward Gibbon, The Decline and Fall of the Roman Empire (XVI):
“Notwithstanding it is probable that Tacitus was born some years before the fire of Rome, he could derive only from reading and conversation the knowledge of an event which happened during his infancy. Before he gave himself to the public, he calmly waited till his genius had attained its full maturity, and he was more than forty years of age, when a grateful regard for the memory of the virtuous Agricola extorted from him the most early of those historical compositions which will delight and instruct the most distant posterity. After making a trial of his strength in the life of Agricola and the description of Germany, he conceived, and at length executed, a more arduous work; the history of Rome, in thirty books, from the fall of Nero to the accession of Nerva. The administration of Nerva introduced an age of justice and propriety, which Tacitus had destined for the occupation of his old age; but when he took a nearer view of his subject, judging, perhaps, that it was a more honorable or a less invidious office to record the vices of past tyrants, than to celebrate the virtues of a reigning monarch, he chose rather to relate, under the form of annals, the actions of the four immediate successors of Augustus.
To collect, to dispose, and to adorn a series of fourscore years, in an immortal work, every sentence of which is pregnant with the deepest observations and the most lively images, was an undertaking sufficient to exercise the genius of Tacitus himself during the greatest part of his life. In the last years of the reign of Trajan, whilst the victorious monarch extended the power of Rome beyond its ancient limits, the historian was describing, in the second and fourth books of his annals, the tyranny of Tiberius; and the emperor Hadrian must have succeeded to the throne, before Tacitus, in the regular prosecution of his work, could relate the fire of the capital, and the cruelty of Nero towards the unfortunate Christians. At the distance of sixty years, it was the duty of the annalist to adopt the narratives of contemporaries; but it was natural for the philosopher to indulge himself in the description of the origin, the progress, and the character of the new sect, not so much according to the knowledge or prejudices of the age of Nero, as according to those of the time of Hadrian. Tacitus very frequently trusts to the curiosity or reflection of his readers to supply those intermediate circumstances and ideas, which, in his extreme conciseness, he has thought proper to suppress.”