Laudator Temporis Acti

Cicero, De Oratore 2.74.299 (tr. E.W. Sutton and H. Rackham):

For instance, we are told that the famous Athenian Themistocles was endowed with wisdom and genius on a scale quite surpassing belief, and it is said that a certain learned and highly accomplished person went to him and offered to impart to him the science of mnemonics, which was then being introduced for the first time; and that when Themistocles asked what precise result that science was capable of achieving, the professor asserted that it would enable him to remember everything; and Themistocles replied that he would be doing him a greater kindness if he taught him to forget what he wanted than if he taught him to remember.

ita apud Graecos fertur incredibili quadam magnitudine consilii atque ingenii 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.

Related post: Remembering and Forgetting.