His artistic purpose was strengthened to something like a prophetic purpose by the environment of his age, the incidents of his life, and the bent of his own intellect. He combats the same enemy as Voltaire waged truceless war upon--the subtle, intangible, omnipresent spirit of insincerity, hypocrisy, and superstition, from which the bigotry and religious oppression of the eighteenth and early nineteenth centuries derived their power. And Gibbon's indebtedness to Voltaire is amazing. There is scarcely a living conception in the _Decline and Fall_ which cannot be traced to that nimble, varied, and all-illuminating spirit. Even the ironic method of the two renowned chapters was prompted by a section in the _Essai sur les Moeurs_. Thus to the theory of Tacitus, the departure from the ancient simplicity of life, Gibbon adds the theory of Zosimus.[9] With Zosimus he affirms that the triumph of Christianism sealed the fate of Rome, and in the Emperor Julian Gibbon finds the same heroic but ill-starred defender of the past, as Tacitus found in the unfortunate Germanicus. This conception informs Gibbon's work throughout, prompting alike the furtive, malignant, or tasteless sketches of the great Pontiffs and the great Caesars, and the finish, the studied care, the vivid detail lavished upon the portraits of their enemies. Half-seriously, half-smiling at his own enthusiasm, he seems to discern in Mohammed, in Saladin, and the Ottoman power, the avengers of Julian and the Rome of the Antonines. And thus Ruskin, inspired by a mood of his great teacher, traces the decline of Venice to its abandonment of Christianism, and Gibbon, influenced by Voltaire and the environment of his age, traces the fall of Rome to the adoption of Christianism. § 5. WHAT IS MEANT BY THE "FALL OF AN EMPIRE"? Underlying both these classes of theories, the retributive and the cyclic, and underlying much of the speculation both of the eighteenth and of the nineteenth century upon the subject, is the assumption that the decay of empires is accidental, or arises from causes that can be averted, or from the operation of forces that can be modified. The mediaeval conception of one empire upon the earth, which yet shall endure forever in righteousness, influences even the mind of Gibbon. He had studied Polybius, and Rome's indefeasible right to the government of the world was the faith which Polybius had announced. And in the hour of Judaea's humiliation and ruin her prophets had still proclaimed a similar hope of everlasting dominion to Israel. But, as the centuries advance, it grows ever clearer that regret or surprise at the passing of empires is like regret or surprise at the passing of youth. Man might as well start once more to discover the elixir of life and alchemy's secrets as hope to found an empire that shall not pass away.