It is probable, however, that he meant to convey some such meaning as this, that those who were both by nature and habit given to the commission of those sins which are committed by the most abandoned of men, could not be completely transformed even by punishment. And yet this is shown to be false from the history of certain philosophers. For who is there that would not rank among the most abandoned of men the individual who somehow submitted to yield himself to his master, when he placed him in a brothel, that he might allow himself to be polluted by any one who liked?
And yet such a circumstance is related of Phædo! And who will not agree that he who burst, accompanied with a flute-player and a party of revellers, his profligate associates, into the school of the venerable Xenocrates, to insult a man who was the admiration of his friends, was not one of the greatest miscreants among mankind? Yet, notwithstanding this, reason was powerful enough to effect their conversion, and to enable them to make such progress in philosophy, that the one was deemed worthy by Plato to recount the discourse of Socrates on immortality, and to record his firmness in prison, when he evinced his contempt of the hemlock, and with all fearlessness and tranquillity of mind treated of subjects so numerous and important, that it is difficult even for those to follow them who are giving their utmost attention, and who are disturbed by no distraction; while Polemon, on the other hand, who from a profligate became a man of most temperate life, was successor in the school of Xenocrates, so celebrated for his venerable character.
Celsus then does not speak the truth when he says “that sinners by nature and habit cannot be completely reformed even by chastisement.”
Source: Against Celsus (New Advent)