Cicero de Oratore 2.74.299-300
English translations by Sutton (Loeb, 1967)

(299) facit enim de se coniecturam; cuius tanta vis ingenii est, ut neminem nisi consulto putet quod contra se ipsum sit dicere; sed ego non de praestanti quadam et eximia, sed prope de volgari et communi vi nunc disputo. 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.
This is because he judges from himself, being a person of such a strong intellect that he cannot imagine anybody saying anything to his own detriment, unless he did so on purpose. But I am not at the moment talking about some outstanding and exceptional ability but about ordinary average capacity. 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.

(300) Videsne quae vis in homine acerrimi ingenii, 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. Sed neque propter hoc Themistocli responsum memoriae nobis opera danda non est neque illa mea cautio et timiditas in causis propter praestantem prudentiam Crassi neglegenda est. Uterque enim istorum non mihi attulit aliquam, sed suam significavit facultatem.
Do you observe what mental force and penetration the man possessed, what power and range of intellect? Inasmuch as his answer brings home to us that nothing that had once been introduced into his mind had ever been able to pass out of it, inasmuch as he  would rather have been able to forget something that  he did not wish to remember than to remember  everything that he had once heard or seen. But this reply of Themistocles must not cause us to neglect the training of the memory, and the exceptional intellectual powers of Crassus must not make us ignore the caution and nervousness in pleading a case that I assigned to myself; for neither Themistocles nor Crassus attributed any competence to me but indicated competence of their own.

Cicero de Oratore 2.86.351-54
(351) Iam istuc quantum tibi ego reliquerim, inquit Antonius, erit in tua potestate. Si enim vere agere volueris, omnia tibi relinquo; sin dissimulare, tu quem ad modum his satis facias videris. Sed ut ad rem redeam, non sum tanto ego, inquit, ingenio, quanto Themistocles fuit, ut oblivionis artem quam memoriae malim; gratiamque habeo Simonidi illi Cio, quem primum ferunt artem memoriae protulisse.
“Oh, as for that,” said Antony, “the amount of memory I shall have left to you will be for you to decide; if you want complete candor, what I leave to you is the whole subject, but if you want me to keep up the pretence, it is for you to consider how you may satisfy our friends here.  But to return to the subject,” he continued, “I am not myself as clever as Themistocles was, so as to prefer the science of forgetting to that of remembering; and I am grateful to the famous Simonides of Ceos, who is said to have first invented the science of mnemonics.

(352) Dicunt enim, cum cenaret Crannone in Thessalia Simonides apud Scopam fortunatum hominem et nobilem cecinissetque id carmen, quod in eum scripsisset, in quo multa ornandi causa poetarum more in Castorem scripta et Pollucem fuissent, nimis illum sordide Simonidi dixisse se dimidium eius ei quod pactus esset pro illo carmine daturum, reliquum a suis Tyndaridis quos aeque laudasset peteret, si ei videretur.
There is a story that Simonides was dining at the house of a wealthy nobleman named Scopas at Crannon in Thessaly, and chanted a lyric poem which he had composed in honor of his host, in which he followed the custom of the poets by including for decorative purposes a long passage referring to Castor and Pollux; whereupon Scopas with excessive meanness told him he would pay him half the fee agreed on for the poem, and if he liked he might apply for the balance to his sons of Tyndaraus, as they had gone halves in the panegyric.

(353) Paulo post esse ferunt nuntiatum Simonidi ut prodiret; iuvenes stare ad ianuam duo quosdam, qui eum magnopere vocarent; surrexisse illum prodisse vidisse neminem. Hoc interim spatio conclave illud, ubi epularetur Scopas, concidisse; ea ruina ipsum cum cognatis oppressum suis interisse. Quos cum humare vellent sui neque possent optritos internoscere ullo modo, Simonides dicitur ex eo, quod meminisset quo eorum loco quisque cubuisset, demonstrator unius cuiusque sepeliendi fuisse. Hac tum re admonitus invenisse fertur ordinem esse maxume, qui memoriae lumen adferret.
The story runs that a little later a message was brought to Simonides to go outside, as two young men were standing at the door who earnestly requested him to come out; so he rose from his seat and went out, and could not see anybody; but in the interval of his absence the roof of the hall where Scopas was giving the banquet fell in, crushing Scopas  himself and his relations underneath the ruins and killing them; and when their friends wanted to bury them but were altogether unable to know  them apart as they had been completely crushed, the story goes that Simonides was enabled by his  recollection of the place in which each of them had  been reclining at table to identify them for separate  interment; and that this circumstance suggested to him the discovery of the truth that the best aid to clearness of memory consists in orderly arrangement.

(354) Itaque iis, qui hanc partem ingenii exercerent, locos esse capiendos et ea, quae memoria tenere vellent, effingenda animo atque in iis locis collocanda; sic fore ut ordinem rerum locorum ordo conservaret, res autem ipsas rerum effigies notaret atque ut locis pro cera simulacris pro litteris uteremur.
He inferred that persons desiring to train this faculty must select localities and form mental images of the facts they wish to remember and store those images in the localities, with the result that the arrangement of the localities will preserve the order of the facts, and the images of the facts will designate the facts themselves, and we shall employ the localities and images respectively as a wax writing tablet and the letters written on it.

Cicero de Oratore 2.87.357-58
(357) verum tamen neque tam acri memoria fere quisquam est, ut non dispositis notatisque rebus ordinem verborum aut sententiarum complectatur, neque vero tam hebeti, ut nihil hac consuetudine et exercitatione adiuvetur. Vidit enim hoc prudenter sive Simonides sive alius quis invenit, ea maxime animis affigi nostris, quae essent a sensu tradita atque impressa; acerrumum autem ex omnibus nostris sensibus esse sensum videndi; quare facillime animo teneri posse, si ea quae perciperentur auribus aut cogitatione etiam commendatione oculorum animis traderentur; ut res caecas et [ab] aspectu[s iudicio] remotas conformatio quaedam et imago et figura ita notaret, ut ea, quae cogitando complecti vix possemus, intuendo quasi teneremus.
Nevertheless hardly anybody exists who has so keen a memory that he can retain the order of all the words or sentences without having  arranged and noted his facts, nor yet is anybody so dull-witted that habitual practice in this will not give  him some assistance.  It has been sagaciously discerned by Simonides or else discovered by some other person that the most complete pictures are formed in our minds of the things that have been conveyed to them and imprinted on them by the senses, but that the keenest of all our senses is the  sense of sight, and that consequently perceptions  received by the ears or by reflection can be most  easily retained in the mind if they are also conveyed  to our minds by the mediation of the eyes, with the result that things not seen and not lying in the field  of visual discernment are earmarked by a sort of image and shape so that we keep hold of as it were by an act of sight things that we can scarcely embrace by an act of thought.

(358) His autem formis atque corporibus, sicut omnibus, quae sub aspectum veniunt, sede opus est; et enim corpus intellegi sine loco non potest. Quare ne in re nota et pervulgata multus et insolens sim, locis est utendum multis inlustribus explicatis, modicis intervallis, imaginibus autem agentibus acribus insignitis quae occurrere celeriterque percutere animum possint. Quam facultatem et exercitatio dabit, ex qua consuetudo gignitur, et similium verborum conversa et inmutata casibus aut traducta ex parte ad genus notatio et unius verbi imagine totius sententiae informatio pictoris cuiusdam summi ratione et modo formarum varietate locos distinguentis.
But these forms and bodies, unlike all the things that come under our view, require an abode, inasmuch as a material object without a locality is inconceivable. Consequently (in order that I may not be tedious on a subject that is well known and familiar) one must employ a large number of localities which must be clear and defined and at moderate intervals apart, and image that are effective and sharply outlined and distinctive, with the capacity of encountering and speedily penetrating the mind; the ability to use these will be  supplied by practice, which engenders habit, and by  marking off similar words with an inversion and alteration of their cases or a transference from species to  genus, and by representing a whole concept by the  image of a single word, on the system and method  of a consummate painter distinguishing the positions  of objects by modifying their shapes.

[DRB 10.2.2009]