A Dispute in the Academy

There was a dispute in the Academy about how to interpret Carneades.

He said that we ordinarily assent to impressions in terms of their persuasiveness. Carneades said this in dialectic with the Stoics, but he also seemed to believe that this is how we should assent to impressions.

If he did believe this, it remained to explain why this is how we should assent to impressions.

The Stoics have an answer for their method of assent in terms of cognitive impressions. They think that nature in its providence arranges things so that this method of assent ensures that we live wise and good lives.

What can the Academics say to show that we should assent in the way Carneades described?

The evidence is extremely limited, but it appears that Clitomachus and Philo (the subsequent heads of the Academy after Carneades) responded in different ways to this question about assent.

Two Ways to Withhold Assent

Cicero provides The evidence for Clitomachus's answer. He says he is quoting from one of Clitomachus's books.

Academica II.98, II.102, II.104

To make sure that no one suspects that I’m making up what I’m going to say, I will use citations from Clitomachus—since he worked with Carneades right up to his old age, and he was a clever man, as you’d expect from a These books have been lost.

"Clitomachus was a Carthaginian, his real name being Hasdrubal, and he taught philosophy at Carthage in his native tongue. He had reached his fortieth year when he went to Athens and became a pupil of Carneades. And Carneades, recognizing his industry, caused him to be educated and took part in training him. And to such lengths did his diligence go that he composed more than four hundred treatises. He succeeded Carneades in the headship of the school, and by his writings did much to elucidate his opinions" (Diogenes Laertius Lives of the Philosophers IV. 10).
Carthaginian, as well as a serious and diligent scholar. There are four books on withholding of assent (98)

Listen to the way these topics are dealt with by Clitomachus in the book he wrote for the poet Gaius Lucilius (he had already written on the same subjects in a book for Lucius Censorinus, the man who shared the consulship with Manius Manilius). He used pretty much these words—I know them well, because the basic primer, as it were, for the subjects currently at issue is contained in this book. At any rate, the book says this. The Academics hold that there are dissimilarities between things, such that some give rise to persuasive impressions, some don’t. Nevertheless, this is not sufficient reason to claim that some things can be perceived while others cannot, because many false impressions are persuasive, but nothing false can be perceived and known. (102)

After expounding these points, Clitomachus added that this. The wise man is said to withhold assent in two ways. One when the meaning is that he gives absolute assent to no "he gives absolute assent to no impression at all."

The Latin is omnino eum rei nulli adsentiri. If the adverb omnino modifies the verb adsentiri, not the object of the verb rei nulli, the translation is "he gives assent to nothing absolutely [or: entirely]." Otherwise, it is "assent to nothing at all." The Loeb translator tries to have it both ways.
impression at all. The other when he restrains himself from replying so as to convey approval or disapproval of something, with the consequence that he neither makes a negation nor an affirmation. Clitomachus holds that he [the wise man] never assents, but that he is guided by persuasiveness, and wherever this confronts him or is wanting he can answer yes or no accordingly. (104)

Because someone who keeps himself from assenting nevertheless wants to move and act, Clitomachus maintained that there are still impressions of the kind that excite us to action; and likewise, there are still responses we can use when questioned on either side, by just following our impressions on the matter, provided we do so without assent. Yet not all impressions of this kind are approved, but only those that aren’t impeded by anything. (104)

Clitomachus distinguishes two ways to withhold assent from every impression. One way is to withhold "absolute assent" from every impression. The other way is to "restrain [one]self from replying" to every impression.

Clitomachus thinks that the wise man withholds assent in the first way but not in the second. He never gives "absolute assent" to any impression, but he does not "restrain himself from replying" to every impression he gets. Sometimes he says 'yes' or 'no' to his impressions, and hence "convey[s] approval or disapproval" in such a way that he follows his impressions in terms of whether he finds them persuasive or not.

What does "absolute assent" mean here?

The answer is not every clear, but the term seems to be a way of talking about the Stoic method of assent. When the Academic forms beliefs, he does not take the himself to do what the Stoics take themselves to do.

What, then, does the Academic take to himself to do?

He tries to do what the "wise man" does.

The wise man does not restrain "himself from replying so as to convey approval or disapproval of something, with the consequence that he neither makes a negation nor an affirmation." Instead, he assents to his impressions in terms of their persuasiveness. If the impression persuasive, he says "yes" to it and is "guided" by its content.

What does "guided" mean here?

One possibility is that to be "guided" by an impression is to believe that the propositional content of the impression is true and to make choices against the background this and other beliefs about how things are.

On this view, the Academic can have false beliefs because an impression can be persuasive but false.

Truth or its Closest Possible Approximation

The evidence for Philo's response depends on Cicero's understanding of Academic practice.

Academica II.7

To be sure, knowledge is always surrounded with difficulties, and the obscurity of the things themselves and weakness of our judgments is such that one can see why the earliest and most learned philosophers lost confidence in their ability to discover what they desired. Still, they didn’t give up, and we won’t abandon our enthusiasm "Carneades points out that he will just follow the probable, what seems to be the case, and that depending on the importance of the matter he will go through certain procedures to make sure that his impression is relatively reliable. It is clear that Carneades' account, first of all, is a dialectical move against a dogmatic objection and thus does not commit him to any view at all. But I also think that it does reflect Carneades' view of how people actually go about gaining an impression they are willing to rely on. And taken this way, it admits of two interpretations. It may be taken in just the sense that this is how human beings in general seem to proceed, or it may be taken in the sense that this is how one ought to proceed if one wants to get a reliable impression, one which if not true, at least has a good chance to be true. Whereas on the first interpretation it is just noted that human beings, as a matter of fact, go about considering matters in a certain way when in doubt, on the second interpretation proper consideration is regarded as conferring some epistemological status on the impression thus arrived at: it has at least a good chance of being true, to be like the truth (versimilis), or else be the truth itself (Cic. [Cicero], Ac. pr. [Academica II] 7; 32; 66; 99; 107). On the other interpretation, the fact that something appears to be the case goes no way to show that it is true; however much it appears to be the case, this does not itself make it any more likely to be true. The probable is just the persuasive" (Michael Frede, "The Skeptic's Two Kinds of Assent and the Question of the Possibility of Knowledge," 214. Essays in Ancient Philosophy, 201-222. University of Minnesota Press, 1987).

Frede's discussion is confusing to understand.

Maybe he means that "on the second interpretation," assenting to persuasive impressions is rational because "the impression thus arrived at ... has at least a good chance of being true... or else be the truth itself,"

If this is what Frede means, then on what he calls "the other interpretation," there is no attempt to explain the rationality of assenting to persuasive impressions by citing this fact about its connection to truth.

On this other interpretation, Frede seems to say that there is no attempt at all to justify the method of assenting to impressions in terms of their persuasiveness that people use to get "an impression they are willing to rely on" As Frede puts the point, "[t]he probable is just the persuasive."

The Greek adjective πιθανόν means "persuasive." Cicero renders the Greek into Latin as probabile, "credible, probable."

"[There is] an etymological and conceptual connection between πιθανόν and πείθεσθαι (to follow; cf. PH [Outlines of Pryrrhonism] I, 230. It is this connection which Cicero tries to preserve when he renders πιθανόν (probable) by probabile to make it correspond to the verb for 'approve' or 'accept' which he likes to use, namely probare (Cic. [Cicero], Ac. pr. [Academica II].99; [Academica II.]139). So the probable quite literally is that which invites approval or assent in the sense in which the skeptic is free to give assent" (Michael Frede, "The Skeptic's Two Kinds of Assent and the Question of the Possibility of Knowledge," 215. Essays in Ancient Philosophy, 201-222).

πιθανόν is a neuter form of πιθανός, "persuasive, plausible"

πείθεσθαι is a form of the verb πείθω, "persuade"

probare is an infinitive form of probo, "approve, esteem, commend"

probabile is a neuter form of probabilis, "likely, credible, probable"

"For believe (πείθεσθαι) is used in different ways. Sometimes it means not resisting but simply following without strong inclination or adherence (as a boy is said to believe his teacher; and sometimes it signifies assenting to something by choice and, as it were, sympathy (as a dissolute man goes along with someone who urges extravagant living)." (Sextus Empiricus, Outlines of Pyrrhonism I.230).
for investigation owing to exhaustion. Nor do our arguments have any purpose other than to draw out or ‘formulate’ the truth or its closest possible approximation by means of arguing on either side. (7)

Cicero is the speaker.

He repeats an understanding of Academic practice.

Whose understanding is it?

There is no talk about "truth or its closest approximation" in the words Cicero quotes from Clitomachus's (now lost) book. This is reason to think Cicero's understanding of Academic practice is not Clitomachus's.

If the understanding is Philo's, the problem for the historian is to construct an interpretation of this understanding.

Academica II.32

Their idea is—and I noticed that you were particularly moved by this—that there are ‘persuasive’ or, as it were, ‘truth-like’ impressions, and this is what they use as their guiding rule both for conducting their lives and in investigation and argument. (32)

Lucullus is the speaker.

He is addressing Cicero as an adherent to the view in the Academy.

Academica II.33

What do you mean by your persuasive impressions? If you mean you rely on what strikes you and seems persuasive at, in effect, first glance, what could be sillier than that? But if they say that they follow impressions that arise from some examination or detailed consideration, they still won’t find any way out. First, because our trust in impressions that don’t differ at all is removed from all of them equally. Second, because they allow that after the wise man has played his part thoroughly by subjecting everything to a meticulous examination, it’s still possible for his impression to be truth-like and yet very far from being true. So even if they do approach the truth for the most part or its closest approximation, as they say they do, they still won’t be able to be confident in their claims. (35)

Lucullus is the speaker.

He argues that the Academics cannot have "confidence" in their method of assent in terms of persuasive impressions because such an impression can "be truth-like and yet very far from being true."

This suggests an interpretation of the Philonian understanding of Academic practice.

Clitomachus tells no story about about how the assent Carneades described is rational (is how we should assent to our impressions) because this method of assent results in beliefs that are true or at least "truth-like." Nor does Clitomachus seem to think it is necessary to tell such a story to justify Academic practice.

We can understand this if we think about the problem the Stoic epistemology pose for the Academy.

The question the Academy faces is whether the Stoic method of assent in terms of cognitive impressions is better than the method in terms of persuasive impressions that Carneades identified as the ordinary way to give assent. Since the Academics think there are no cognitive impressions, Clitomachus's answer is that the Stoic method is not better and thus that the Academic should keep saying "yes" or "no" to their impressions in terms of the persuasiveness of these impressions. Clitomachus thinks that this way of assenting to form beliefs is rational and that the Stoics have not provided a reason to abandon it for their new method in terms of cognitive impressions.

Philo, it seems, thought that more was necessary to establish that the method Carnaedes described is rational.

Philo's view, on this interpretation, is that the method Carneades described for finding an impression to be persuasive is a method for finding that the impression is "like" the truth if it is not the truth and that this fact about the method is what makes it a rational method for giving assent and forming beliefs.

If this interpretation is right, Philo does more than Clitomachus. To justify the method of assent Carneades described, Philo takes a position similar to the one the Stoics take to justify their method of assent.

The Stoics think that their method of assent is rational because nature in its providence arranges things so that we have the truth if we follow this method. It is not clear what view if any Philo puts in the place of this Stoic view about nature, but he says that the method of assent Carneades described is rational because the propositional contents of persuasive impressions are at least "like" the truth if they are not the truth.

Academica II.66

How could I not desire to find the truth when I rejoice if I find something truth-like. (66)

Cicero is the speaker.

Academica II.99

Your wise man also follows persuasive impressions in many cases—i.e., impressions that aren’t grasped, perceived, or assented to, but are truth-like. If he didn’t approve them, his whole life would be undermined. Here’s one case: when he steps into a boat, does the sage grasp in his mind and perceive that he is definitely going to arrive? How could he? Still, should he set out from here to Puteoli, thirty stades away, in a tested vessel, with a good helmsman, and in calm weather like this, he would have the persuasive impression that he will arrive there safely. (99)

Cicero is the speaker.

He thinks that because the Stoics sage is not omniscient, he will sometimes follow persuasive impressions.

It is unclear that the Stoics themselves think this.

Academica II.107

Panaetius (185-110 BCE)
He introduced Stoic doctrines to Rome.
Panaetius (who is pretty much the best of the Stoics, in my view) says that he has doubts about something that every other Stoic except him thinks is quite certain, namely, that the responses of entrail-diviners, auspices, oracles, dreams, and prophecies are true. So he restrains his assent about this. But if he can do that even in matters his teachers hold to be certain, why can’t the wise man do the same in every other case? Is there really any assertion that the wise man can approve or disapprove but can’t doubt? Do you really think that you can do this at any stage you like in a sorites argument, but he can’t come to a similar halt in every other case, especially when it’s open to him to follow unimpeded truth-like impressions without assent? (107)

Cicero is the speaker.

He is replying to the inactivity argument the Stoics pressed against the Academics. He argues that the wise man need not be inactive if he does not assent because he can "follow unimpeded truth-like impressions."

Academica II.127

The process of investigation into the greatest (if also most hidden) matters has its own delight; and if we come across something that strikes us as truth-like, our minds are suffused with a thoroughly humane pleasure. So both your wise man and ours will investigate these questions [about the size of the sun and other such physical matters], but yours to assent, believe, and affirm, ours with the fear of forming rash opinions and the thought that things are going wonderfully for him if he finds something truth-like in questions of this sort. (127)

Cicero is the speaker.

He says that the Academic fears having "rash opinions."

His point is not very clear.

He might mean that the Academic avoids not opinions themselves but rash opinions and that to do this, he assents to persuasive impressions so that his beliefs are at least "truth-like" if they are not true.

This, though, is not the only possibility.

He might mean that the Academic avoids opinions altogether. Opinions are irrational and the result of rash assent. The Academic "follows" the impressions he finds persuasive. In doing this, he "finds something truth-like."

The reason for this unclarity, I think, is that Cicero does not fully understand the positions he is talking about.

Academica II.148

  But what does Catulus think, and Hortensius?
  What do I think? I am returning to my father’s view, which he at least said was Carneades’s. That is, while I don’t think that anything can be perceived, I still reckon that the wise man will assent to something he hasn’t perceived—that is, hold opinions—but in such a way that he understands that it is an opinion and realizes that nothing is Socrates, in the Gorgias, expresses a similar but more measured view.

"My story, Callicles, is ever the same, that I do not know how these things are, and yet all of whom I have encountered, before as now, no one has been able say otherwise without making himself ridiculous. So once more I set down these things" (Gorgias 509a).
can be comprehended and perceived. And therefore although I ‡ ... ‡ with their rule of ἐποχή [suspension], I assent emphatically to that second view that nothing can be perceived.
  I’m glad to have your view, Catulus, and I don’t greatly reject it.
  But what are your thoughts, Hortensius?
  Away with it.
  I’m with you, since that’s a view that suits the Academy. (148)

Cicero asks Catulus and Hortensius what they think.

The text is corrupt. I does not show whether "agree" or "do not agree" is in the original.

Whose view does Catulus express?

The tone Catulus employs to describe his view is not present in Cicero's report of what Clitomachus says the wise man does to "convey approval or disapproval." Catulus expresses a confidence that contrasts with the humbleness of of simply saying "yes" and "no" to impressions and thus following them in terms of their persuasiveness.

A Different Interpretation of the Dispute

Charles Brittain gives a standard interpretation the dispute in the Academy we have been thinking about.

LAS = Philo of Larissa: The Last of the Academic Sceptics (2001)
OAS = On Academic Scepticism (2006)
POL = "Philo of Larissa" (2006)
Brtttain is the author of 2001 Oxford publication Philo of Larissa: The Last of the Academic Sceptics and of the 2006 Hackett publication On Academic Scepticism. Brittain is also the author of the Stanford Encyclopedia of Philosophy entries on the Academic philosophers Arcesilaus and Philo of Larissa.

Brittain distances himself from the coherence of “radical scepticism.”

“The philosophical sense of the distinction between these two groups [the Clitomachians and the Philonians] has proved difficult to capture” (LAS, 16). “The manner in which it is held best to spell out this distinction [that defines the Clitomachian position] is not significant here" (LAS, 75). “It remains very controversial whether there is a coherent distinction to be made between approving a view [the Clitomachian position] and assenting to its truth [the position of the Stoics and the Philonians]” (POL). “It is perhaps unclear how we should (or even can) make sense of this [Clitomachus’s] position” (OAS, xxvii).

Why, then, is Brittain so sure that Clitomachus has this view?

He thinks that Cicero correctly understands Clitomachus's view.

Here the primary text Brittain uses to support his interpretation:

"I believe Clitomachus when he writes that Carneades had accomplished an almost Herculean labour in that he had driven assent—i.e., opinion and rashness—from our minds, as one would drive out a wild and savage monster" (Academica II.108).

In these remarks, as Brittain interprets them, Cicero takes Clitomachus to understand Carneades to show how we can live without any beliefs.

If Brittain's interpretation of Cicero is correct, we still need to know that Cicero did not misunderstand Clitomachus.

Do we know this?

Carneades set out procedures for assenting to impressions in terms of their persuasiveness, and it might be that Clitomachus thought this was how Carneades showed how to avoid the hasty thinking.

This interpretation is consistent with the explanation of the two ways to withhold assent that Cicero says he quotes from Clitomachus.

One might object that Cicero is not likely to have misunderstood Clitomachus, but this must be set against the implausibility of the philosophical view Brittain's interpretation attributes to Clitomachus.
In this interpretation of the dispute, Brittain sets out two positions. The first is how Clitomachus is supposed to understand Carneades. Brittain calls Clitomachus's interpretation "radical scepticism." Brittain's second position is how Philo is supposed to understand Carneades. Brittain calls this interpretation "mitigated scepticism."

Here is Brittain's explanation and defense of his interpretation.

"On the Clitomachian view of Carneades, however, while the Academics will ‘follow’ persuasive impressions, or ‘approve’ them, they will not assent to them: that is the Academics will act on such impressions, without taking them to be true" (LOS, 16; emphasis in the original).

"[Although] the Academic does not hold dogmatic beliefs asserting the truth of a given proposition, it is open to him to ‘follow’ or ‘approve’ impressions which are ‘persuasive’ to him, in the sense that he may act on such impressions as if they were true without committing himself to their truth" (LOS, 74; emphasis in the original).

"Clitomachus argued [that] we should distinguish between ‘assenting’ to an impression in the Stoic sense and ‘approving’ it ..., i.e., acting on it as if we took it to be true. The Academic thus gives his approval to the impressions he acts on—he ‘follows’ persuasive impressions—but does not assent to them" (OAS, xxvii).

The view Brittain attributes to Clitomachus is implausible philosophically. That does not mean Clitomachus did not hold it, but it does mean Britain must give us strong evidence that Clitomachus did hold it.

Clitomachus is supposed to think he "follows" impressions but does not "assent" to them. Brittain thinks that this is what Clitomachus is saying when he describes how the "wise man is said to withhold assent in two ways."

Brittain understands Clitomachus to think that assenting to an impression is taking its propositional content to be true and to think the wise man does not do that. Instead forming beliefs by assenting to certain impressions, he only follows these impressions. In doing this, he acts as if the propositional contents of these impressions are true.

This gives Clitomachus an extremely surprising view. We can understand how someone in a play acts as if certain propositions are true, but it is harder to understand how this could be true for every impression someone gets. Someone like this has no beliefs. He does not represent the world as being a certain way and choose his actions by thinking about how what he does will change the world as he understands it. This means he has no mind.

What is "the Stoic sense" Brittain is talking about?

Maybe he means that Clitomachus distinguished between two senses of the Greek counterparts of the word assent. On one sense, it means "commit oneself the truth of the propositional content of the impression." On the other, it means "act as if one commits oneself to the truth of the propositional content of the impression."

The Greek words do not have these senses. This, again, does not mean that Clitomachus did not say that they did, but Brittain has to give us strong evidence before it is reasonable for us to accept this interpretation.

Clitomachus's remarks on withholding assent are not strong evidence for the view Brittain attributes to him. Clitomachus seems to be talking about "ways" of doing things, not the senses of words.

"[W]hen the Academics draw their notorious conclusions about the unattainability of knowledge and the irrationality of forming beliefs, they are maintaining only that these conclusions are currently ‘persuasive’: they are not committed to the truth of these views or of the arguments that support them. But in that case, the Clitomachian Academics do not believe, e.g., that nothing can be known, or, at least, they do not believe it in the sense implied by Stoic assent. It is perhaps unclear how we should (or even can) make sense of this position, but it is plain that Clitomachus did not subscribe to the form of dogmatic, negative scepticism suggested by his advocacy of [that there are no cognitive impressions and that it is irrational to assent to noncognitive impressions]. For this reason, it is appropriate to call this Academic position ‘radical scepticism'" (OAS, xxvii).

“[On one] definition, a sceptic is someone who positively evaluates abstention from all belief; scepticism in that sense is idiotic” (Harty Field, “Apriority as an Evaluative Notion,” 127. New Essays on the A Priori, 117-149. Oxford University Press, 2000). The view Brittain attributes to the "Clitomachean Academics" is more than "radical."

"Clitomachus distinguishes between dogmatic ‘assent’ and the sceptic’s ‘approval’ by identifying two kinds of suspension of assent. The first [is] the refusal to accept that an impression is true (the negative correlative of dogmatic assent). The second kind of suspension of assent is universal: a flat refusal to accept any impression in any way at all. The Clitomachian Academic rejects the second kind of suspension in approving or ‘assenting’ to persuasive impressions; but he continues to suspend assent in the first sense because he acts on persuasive impressions without thereby taking them to be true" (OAS, note 154 on 61).

Now Brittain is talking about "kinds of suspension of assent," not senses of words.

Words have senses. They are their meanings. The word 'cape,' for example, has two senses. It can mean "article of clothing worn over the shoulder" like Batman wears or "strip of land jutting out to sea."

What is the "second kind of suspension of assent"?

It is to suspend assent by not "accept[ing] an impression in any way at all."

Brittain understands Clitomachus to think that because the wise man follows but does not assent to impressions, he does not "refus[e] to accept any impression in any way at all."

"The second position—which [is adopted by Philo and probably Metrodorus] we can call ‘mitigated scepticism’ to distinguish it from Clitomachus’—accepts inapprehensibility, but maintains that it does not follow that we should reject all assent. Hence, these Academics reject the conclusion that it is irrational to assent to anything. But they also characterize the beliefs they do form as ‘opinions’ to mark the fact that, despite their rational grounds for holding them, they are explicitly rational beliefs, i.e., they do not amount to apprehensions. If this is right, the mitigatedly sceptical position is one that advocates forming reasonable beliefs in conditions of uncertainty—and since, in their view, conditions are always uncertain owing to the nonexistence of cataleptic impressions, it also claims that reasonable beliefs are the best we can hope to attain" (OAS, xxvii-xxix).

Philo is supposed to have held what Brittain calls "mitigated scepticism."

The mitigated skeptic assents to impressions. He commits himself to their propositional contents. This is a big step away from the radicalness of radical skepticism because the mitigated skeptic has beliefs.

The mitigated skeptic thinks that assenting and forming beliefs in certain situations is rational. This too is a big step away from the radicalness of radical skepticism. The radical skeptic says that assent and belief is irrational.

(The radical skeptic does not assent, so he does not assent to the impression that assent and belief is irrational. He follows this impression and acts as if assent and belief is irrational. How does he act this way?)

"The crucial difference was the change from the purely subjective Clitomachian interpretation of the pithanon [πιθανόν]--under which there is no inference from what is persuasive to what is true-- to the Philonian/Metrodorians’ quasi-objective use of it as evidence for the truth" (LOS, 16).

“These mitigated skeptics thus took the persuasiveness of perceptual impressions under the right perceptual and coherence conditions as defeasible, but rational, evidence for their truth, rather than as merely the ground for their acceptance" (POL; emphasis in the original).

"The difference between this position and Clitomachus’ is difficult to discern in practice, because both groups used the same criteria for evaluating impressions and philosophical theses. So both groups use the subjective ‘clarity’ of their perceptual impressions and their coherence with other impressions to grade their degree of ‘persuasiveness,' and both use the Academic method of arguing on either side of philosophical questions to examine which are more ‘persuasive.’ But they differ in that the mitigated sceptics will assent to persuasive impressions or claims when the evidence supporting them is sufficiently strong—and they assume that persuasiveness under the appropriate conditions does provide evidence for the truth" (OAS, xxvii-xxix).

What does Brittain mean with his talk about truth and evidence here?

I am not sure.

Here is one possibility. Clitomachus and Philo disagreed about whether the persuasiveness of an impression is evidence for the truth of its propositional content. Clitomachus said it was not evidence.

"[Because Carneades] himself too requires a criterion for the conduct of life ..., he is practically compelled on his own account to frame a theory about it, and to adopt both the persuasive impression and the impression which is at once persuasive and irreversible and tested" (Sextus Empiricus, Against the Logicians I.166).

"[J]ust 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 question each on the testimony of the others,--so likewise, says Carneades, in trivial matters we employ as criterion only the persuasive impression, but in greater matters the irreversible, and in matters which contribute to happiness the tested impression (Sextus Empiricus, Against the Logicians 1.184).
It can seem clear that Clitomachus is right. Think about what Sextus Empiricus says about Carneades.

In ordinary life, when the question is about a small matter, we are content to take the word of a single witness. Given that the degree of importance we attribute to the matter is small and that we have no reason to be suspicious, we accept the testimony from the witness that such-and-such happened. This makes the impression that such-and-such happened persuasive, but the evidence for the truth of the propositional content of the persuasive impression is not that the impression is persuasive. It is that the witness told us that it happened.

Maybe Brittain is not really talking about evidence.

Consider the example again. Let I be the impression that such-and-such happened. I is persuasive for S because S accepts the testimony and infers from this testimony that the propositional content of I is true.

According to Brittain, on this reading of his interpretation, Clitomachus does not think that an impression is ever persuasive for someone because he has inferred that propositional content of the impression is true.

Brittain thinks, it seems, that this understanding of Clitomachus follows from the fact that he does not take the propositional contents of the persuasive impressions he follows to be true. He only acts as if they are true.

"The Stoics conceived of assent as a unitary notion—to assent to an impression (or proposition) is simply to take it to be true. Clitomachus had identified a non-dogmatic form of assent, ‘following’ or ‘approving’ what is persuasive, where to ‘follow’ what is persuasive’ is to act on an impression as if it were true, but without any commitment to its truth in reality. The Philonian/Metrodorians in turn recognize a third form of assent: provisionally taking the impression to be true" (LOS, 88; emphasis in the original).

"Philonian/Metrodorian beliefs are provisional in two respects: they incorporate the recognition that they may be false, and they are explicitly related to a body of (subjective) evidence, which in virtue of the thesis of acatalepsia [ἀκαταληψία], is avowedly incomplete. The second feature here explains the first; however strong the conditions on giving one's assent to a belief may be, the possibility of further evidence against the belief remains. The use of the Stoic term 'opinion' by the Philonian/Metrodorians is thus explained by a simple aspect of such beliefs: they are (wittingly) defeasible" (LOS, 87; emphasis in the original).

If we understand Brittain’s interpretation is understood along the lines I have been suggesting, his point is that whereas the Clitomachian does not accept that he has a reason to believe in the truth of the propositional content of an impression if the impression is persuasive for him, the Philonian/Metrodorian accepts the validity of this inference but also “mitigates” his acceptance in a certain way. The Philonian/Metrodorian accepts that he has a reason to believe in the truth of the propositional content of an impression if the impression is persuasive for him, but he takes the belief he forms in the truth of this proposition to be "provisional."

By "provisional" here, Brittain means what contemporary epistemologists more commonly call "defeasible."

Socrates seemed to show that relative to any belief about the matters he discussed, the believer has a reason to withdraw it. Britain takes the Philonian/Metrodorian to recognize that this could happen to him.

"The fact that [mitigated sceptics] take the thesis that nothing can be known to be a paradigm for rational belief or ‘opinion’ itself indicates the depth of their disagreement with more radical sceptics like Clitomachus. Their view implies, after all, that they have bought into the Stoic theories of psychology, impressions, perception, etc., because they think that they have adequate rational grounds to believe that the Stoic definition of apprehension is correct even though its conditions can never be met" (OAS, xxx; emphasis in the original).

One "provisonal" or "defeasible" belief the mitigated skeptic possesses is that nothing can be known.

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