The Theory of Persuasive Impressions
Carneades talks about "persuasive impressions." In this, he seems to have in mind the ordinary way in which human beings think about things and form beliefs once they have sufficient evidence relative to the importance they attribute to the matter. Unless something intervenes to stop them, they ordinarily consider the matter in question until they have sufficient evidence to decide it one way or another given the importance they attach to it. Once they have this evidence, they ordinarily accept that the issue is as their thinking has revealed it to be.
Cicero (106 - 43 BCE)
"[W]hen [the Stoic] Antipater used ... to say that one who asserted that nothing could be perceived might yet consistently say that this single fact could be perceived, namely that nothing else could, Carneades with greater acumen used to oppose him; he used to declare that this was so far from being consistent that it was actually grossly inconsistent: for the man who said there was nothing that was perceived made no exception, and so not even the impossibility of perception could itself be grasped and perceived in any way, because it had not been excepted" (Academica II.28-29).
"For they [the Academics] hold (and this in fact, I noticed, excites your school [the Academy] extremely) that something is 'probable,' or as it were resembling the truth, and that this provides them with a canon of judgement both in the conduct of life and in philosophical investigation and discussion" (Academica II.32).
The Greek adjective πιθανόν means "persuasive." Cicero renders the Greek into Latin as probabile, "credible, probable." "... bring forward 'probable presentation (φαντασία πιθανὴ),' or 'probable and unhampered presentation,' as Carneades held, or something else, as a guide for you [the Academics] to follow..." (Academica II.33-34).
"I [Cicero] shall take it from Clitomachus, who was a companion of Carneades quite until old age, a clever fellow as being a Carthaginian, and also extremely studious and industrious. There are four volumes of his that deal with the withholding of assent, but what I am now going to say has been taken from Volume One. Carneades holds that there are two classifications of impressions, which under one are divided into those that can be perceived and those that cannot, and under the other into those that are probable and those that are not probable; and that accordingly those impressions that are styled by the Academy contrary to the senses and contrary to perspicuity belong to the former division, whereas the latter division must not be impugned; and that consequently his view is that there is no impression of such a sort as to result in perception, but many that result in a judgement of probability. For it is contrary to nature for nothing to be probable, and entails that entire subversion of life of which you, Lucullus, were speaking; accordingly even many impressions must be deemed probable, if only it be held in mind that no impression has such a character as a false impression could not also have without differing from it at all. Thus the wise man will make use of whatever apparently probable impression he encounters, if nothing presents itself that is contrary to that probability, and his whole plan of life will be charted out in this manner. In fact [I think that] even the person whom your school [the Stoics] brings on the stage as the wise man follows many things probable, that he has not grasped nor perceived nor assented to but that possess verisimilitude; and if he were not to approve them, all life would be done away with" (Academica II.99-100).
Sextus Empiricus (2nd or 3rd century CE)
"[Carneades] too, himself requires a criterion for the conduct of life and for the attainment of happiness, he is practically compelled on his own account to frame a theory about it, and to adopt both the plausible impression (πιθανὴν φαντασίαν) and that which is at once plausible and irreversible and tested" (Against the Logicians I.166-167).
"[T]hat which appears true, and appears so vividly, is the criterion of truth according to the School of Carneades. ... [T]he Academics called [these impressions] 'plausible'" (Against the Logicians I.171-175).
"[T]he Academic forms his judgement of truth by the concurrence of impressions, and when none of the impressions in the concurrence provokes in him a suspicion of its falsity he asserts that the impression is true. ... Still more trustworthy than the irreversible impressions and supremely perfect is that which creates judgement; for it, in addition to being irreversible, is also 'tested.' What the distinctive feature of this impression is we must next explain. Now in the case of the irreversible impression it is merely required that none of the impressions in the concurrence should disturb us by a suspicion of its falsity but all should be apparently true and not improbable; but in the case of the concurrence which involves the 'tested' impression, we scrutinize attentively each of the impressions in the concurrence..." (Against the Logicians I.178-182).
"For all these factors together form the criterion—namely, the probable impressions, and that which is at once both probable and irreversible and besides these that which is at once probable and irreversible and tested. And it is because of this that, just as in ordinary life when we are investigating a small matter we question a single witness, but in a greater matter several, and when the matter investigated is still more important we cross-question each of the witnesses on the testimony of the others,—so likewise, says Carneades, in trivial matters we employ as criterion only the probable impressions, but in greater matters the irreversible, and in matters which contribute to happiness the tested impressions. Moreover, just as they adopt, they say, a different impressions to suit different cases, so also in different circumstances they do not cling to the same impressions. For they declare that they attend to the immediately probable in cases where the circumstances do not afford time for an accurate consideration of the matter. A man, for example, is being pursued by enemies, and coming to a ditch he receives a impressions which suggests that there, too, enemies are lying in wait for him; then being carried away by this impressions, as a probability, he turns aside and avoids the ditch, being led by the probability of the impressions, before he has exactly ascertained whether or not there really is an ambush of the enemy at the spot. But they follow the probable and tested impressions in cases where time is afforded for using their judgement on the object presented with deliberation and thorough examination. For example, on seeing a coil of rope in an unlighted room a man jumps over it, conceiving it for the moment to be a snake, but turning back afterwards he inquires into the truth, and on finding it motionless he is already inclined to think that it is not a snake, but as he reckons, all the same, that snakes too are motionless at times when numbed by winter’s frost, he prods at the coiled mass with a stick, and then, after thus testing the impressions received, he assents to the fact that it is false to suppose that the body presented to him is a snake" (Against the Logicians I.184-188).