Letter XVIII. To Pope Damasus.
The letter is noticeable for the evidence it affords of the thoroughness of Jerome's studies. Not only does he cite the several Greek versions of Isaiah in support of his argument, but he also reverts to the Hebrew original. So far as the West was concerned he may be said to have discovered this anew. Even educated men like Augustine had ceased to look beyond the LXX., and were more or less aghast at the boldness with which Jerome rejected its time-honored but inaccurate renderings.
The letter also shows that independence of judgment which always marked Jerome's work. At the time when he wrote it he was much under the sway of Origen. But great as was his admiration for the master, he was not afraid to discard his exegesis when, as in the case of the seraphim, he believed it to be erroneous.
Source: Letters (New Advent)