Cicero, de Oratore 2.74:
It is said among the Greeks that Themistocles was endowed with a certain incredible greatness of thought and intellect. It is said that once a certain learned man, one of the most educated of his time, approached him and promised to teach him the art of memory, which was then first becoming popular. When Themistocles asked what that could do, the teacher responded that it could make one remember everything. Themistocles responded in turn that the teacher would be doing him a much greater favor if he taught him to forget rather than remember what he wanted. Do you see what force, what a great and powerful mind there was in this man of such sharp intellect? He responded in such a way that we can understand that nothing was ever able to flow out of his mind when once it had gotten in there, since indeed he thought it preferable to be able to forget what he did not want to remember than to remember what he had heard or seen only once.
Ita apud Graecos fertur incredibili quadam magnitudine consili atque ingeni Atheniensis ille fuisse Themistocles; ad quem quidam doctus homo atque in primis eruditus accessisse dicitur eique artem memoriae, quae tum primum proferebatur, pollicitus esse se traditurum; cum ille quaesisset quidnam illa ars efficere posset, dixisse illum doctorem, ut omnia meminisset; et ei Themistoclem respondisse gratius sibi illum esse facturum, si se oblivisci quae vellet quam si meminisse docuisset. Videsne quae vis in homine acerrimi ingeni, quam potens et quanta mens fuerit? Qui ita responderit, ut intellegere possemus nihil ex illius animo, quod semel esset infusum, umquam effluere potuisse; cum quidem ei fuerit optabilius oblivisci posse potius quod meminisse nollet quam quod semel audisset vidissetve meminisse.