John Stuart Mill (1806-1873), On Liberty
The greatest orator, save one, of antiquity, has left it on record
that he always studied his adversary's case with as great, if not
still greater, intensity than even his own. What Cicero practised as
the means of forensic success requires to be imitated by all who study
any subject in order to arrive at the truth. He who knows only his own
side of the case, knows little of that. His reasons may be good, and
no one may have been able to refute them. But if he is equally
unable to refute the reasons on the opposite side; if he does not so
much as know what they are, he has no ground for preferring either
opinion. The rational position for him would be suspension of
judgment, and unless he contents himself with that, he is either led
by authority, or adopts, like the generality of the world, the side to
which he feels most inclination. Nor is it enough that he should
hear the arguments of adversaries from his own teachers, presented
as they state them, and accompanied by what they offer as refutations.
That is not the way to do justice to the arguments, or bring them into
real contact with his own mind. He must be able to hear them from
persons who actually believe them; who defend them in earnest, and
do their very utmost for them. He must know them in their most
plausible and persuasive form; he must feel the whole force of the
difficulty which the true view of the subject has to encounter and
dispose of; else he will never really possess himself of the portion
of truth which meets and removes that difficulty.
Cicero, De Oratore
2.24.102 (tr. H. Rackham):
It is my own practice to take care that every client personally instructs me on his affairs, and that no one else shall be present, so that he may speak the more freely; and to argue his opponent's case to him, so that he may argue his own and openly declare whatever he has thought of his position. Then, when he has departed, in my own person and with perfect impartiality I play three characters, myself, my opponent and the arbitrator.
equidem soleo dare operam, ut de sua quisque re me ipse doceat et ut ne quis alius adsit, quo liberius loquatur, et agere adversarii causam, ut ille agat suam et, quidquid de sua re cogitarit, in medium proferat. itaque cum ille discessit, tres personas unus sustineo summa animi aequitate, meam, adversarii, iudicis.
Whereas if there has really ever been a person who was able in Aristotelian fashion to speak on both sides about every subject and by means of knowing Aristotle's rules to reel off two speeches on opposite sides on every case, or in the manner of Arcesilas and Carneades argue against every statement put forward, and who to that method adds the experience and practice in speaking indicated, he would be the one and only true and perfect orator.
sin aliquis exstiterit aliquando qui Aristotelio more de omnibus rebus in utramque sententiam possit dicere et in omni causa duas contrarias orationes praeceptis illius cognitis explicare, aut hoc Arcesilae modo et Carneadis contra omne quod propositum sit disserat, quique ad eam rationem adiungat hunc usum exercitationemque dicendi, is sit verus, is perfectus, is solus orator.