Philosophy

Stoicism assumes that knowledge can be obtained through individual and collective use of reason. Knowledge however is a manifestation of sense-objects. Truth is but a collective impression to stimulus. Now, the mind has the ability to distinguish accurate and inaccurate impressions (also called graspable and non-graspable presentations). An accurate impression is a stimulus derived from an existing object, not necessarily material, that has become an accurate representation of the object itself. An inaccurate impression is an impression whose origin is unknown or vague (Cicero, 236l).

An accurate impression is almost always the necessary criterion of knowledge because reality is materially established. The imprint of a certain object to ones mind becomes a part of the sense experience, and therefore part of reality. However, an accurate impression is not knowledge yet. Knowledge is a generalized, accepted accurate impression which cannot be overturned by argument. This is equivalent to the general laws of nature.

This was not the case with Carneades. Carneades argued that every true presentation has a corresponding false presentation. Every accurate impression  whether it constitutes knowledge  has an associated false impression (not to be confused with inaccurate impression). Carneades argued that it is impossible to possess any criterion of truth. Now, suppose that there is a criterion for the sake of argument, such criterion must exist either in reason or conception. However, because reason depends on conception, and conception on sensation, it would be impossible to determine the rational efficacy of sensations. Sensations, for the most part, are far removed from absolute truth (Cicero, 242lg). Man is therefore driven to error by sensation. In short, neither reason nor sensation is a criterion of truth.

Although it is impossible to find a single and efficient criterion of truth, one can establish probabilities of varying levels. Some sensations are more efficient than others. Indeed, based from sense experience, sensations are not alienated from other forms of sensations. They mingle in an ocean of impressions.  The more pronounced a group of impression is, the more it is closer to truth. The combination with the highest probability must be the nearest approximation of truth.

For Carneades, it is impossible to know something  that reality is an imagery of the human mind. However, when the Stoics asked Carneades for some criteria for life and happiness, he was forced to take a position. Carneades argued that knowledge is not a requisite for efficiency, that efficiency itself is not a requirement of action. However, in order to live a reasonable life, man must assent to the conceivable presentation (pithanon). Pithanon is a general criterion of action rather than knowledge. According to Cicero, pithanon (pithane phantasia) is a formalistic digression of action itself it is neutral to knowledge because it does not presuppose any truth value.

However, for Carneades, pithanon can be a criterion (but not a necessary) for both knowledge and action. Note that impressions are grouped. The more pronounced a group of impression is, the more it is closer to truth. Any plausible presentation can be a part of truth but never its empirical manifestation. Pithanon is a performance rather than a suspension of judgment.

Pithane Phantasia as Original Criterion
Carneades predecessor, Arcesilaus argued that it is impossible to know anything about the real world, that knowledge itself is a vague ideal. Because nothing can be known about the world, it is necessary for the individual to suspend judgment. However, this does not mean the abandonment of reason. Reason is the criterion for conducting ones life  the guide to correct action. The individual need not assent to anything.

Carneades found Arcesilaus propositions confusing. If individuals need not assent to anything, then reason would be considered (eulogon) false criterion. As Annas correctly observed

His portrayal of eulogon as a criterion by which to regulate ones choices seems not to cohere with his insistence that one need not assent to anything  whether these were intended as parts of a single account of human action, or as separate but complementary claims. He seems concerned, on the one hand, to present the sceptic as having the resources for a life that is both distinctively human and worth living yet, by subtracting assent from his model of behavior, he appears to cut off that possibility. Of course, it is always conceivable that he had some way, now lost to us, of rendering the whole picture coherent  (67).

Now, pithane phantasia or probabile impression is the only efficient criterion for action. It does not indicate or describe the state of affairs of any object. It is a general guide to correct action. Epistemologically, pithane phantasia is a plausible presentation. Carneades accepted the idea that the truth of an impression comprises in its association with the object. However, Carneades rejects the possibility that one can fully determine whether there is indeed an association between the object and the impression.

A repetitive and firm presentation appears to be both true and unambiguous. Such impression, using pithane phantasia, has a high probability of being true. Suspension of judgment is applicable only when one examines the truth of an impression (but not its manifesting truth). Because unqualified assent is not reliable, one is forced to accept the veracity of plausible representations, with reservations. In short, plausible impressions justify ones competent assent therefore serve as a criterion for action.

It is in this sense that Arcesilaus criterion eulogon is an insufficient criterion for action. As Annas noted
In asserting that the eulogon is the criterion of action  and that assent is not necessary for action, Arcesilaus already seems to be in trouble the very making of assertions, whether or not they are in themselves convincing, is inconsistent with a posture of universal suspension of judgment. Indeed, the assertion that nothing can be known, and even the recommendation that one suspend judgment about anything put Arcesilaus in difficulty (68-69).

The failure of the eulogon criterion to effectively assess the state of affairs of impressions was but a failure on the part of Arcesilaus to perceive the inherent contradiction in his model. Carneades corrected the mistake by assuming that it is possible to assent to anything, that there is a separation between inherent truth and apparent truth, and that plausible impressions can become a guide to correct action.

It is in this sense that pithane phantasia is the first complete definitive criterion of action. It is complete in the sense that it is a summative expansion of the eulogon criterion (with the difficulty of the assent proposition). It is definitive in the sense that it allows flexibility of rational interpretation.

Conclusion
Carneades developed the concept of pithane phantasia to correct Arcesilaus conceptual errors and to provide a more subtle but complete criterion for action.