Suetonius,
The Lives of the Twelve Caesars…
“ Had Sejanus now been governed by common prudence and moderation, he might have attained to the accomplishment of all his wishes; but a natural impetuosity of temper, and the intoxication of power, precipitated him into measures which soon effected his destruction. As if entirely emancipated from the control of a master, he publicly declared himself sovereign of the Roman empire, and that Tiberius, who had by this time retired to Capri, was only the dependent prince of that tributary island. ”
