Philosophy 143 Lecture Notes

The Epistemology of the Stoics

The Problem of False Presentations

As discussed in the context of Epicurean epistemology, many of the ancient Greek philosophers were suspicious of sense-perception as a source of knowledge (as opposed to opinion). The basic problem is simply that experience shows that the senses often provide conflicting information in different contexts. The Stoics were acutely aware of these problems but nonetheless maintained that sense-perception is a reliable source of knowledge, under the right conditions. They disagreed with the Epicureans in maintaining that some sense-perceptions depict their objects adequately while others do not, and consequently that some judgments based on sense-perception are true, while others are false. Cicero tells us:

Arcesilaus [the Academic skeptic] attacked Zeno [the founder of Stoicism] because, while he himself said that all sense-perceptions were false, Zeno said that some were false, but not all. Epicurus was afraid that, if one sense-perception were false, none would be true; so he said that all sense-perceptions were messengers of the truth. [Epicurus did not] show much cleverness; in order to ward off a minor blow, he opened himself up to a more serious one." (On the Nature of the Gods, 1.70)
The problem faced by Zeno and his followers was to give a criterion whereby it would be possible to distinguish those sense-perceptions which are "messengers of the truth" from those which give a false view of the world. The Epicurean criterion is the sense-perception as such, which obviates the need for the difficult task of defending a way of distinguishing sense-perceptions with respect to their truthfulness.


This task was particularly important because the Stoics considered sense-perception as the starting-point of all knowledge. At birth, a person is devoid of any conceptions of things, and it is only sense-perception which allows us to build up a stock of conceptions on which we exercise our rationality. As Sextus put it:

For every thought comes from sense-perception or not without sense-perception and either from direct experience or not without direct experience. Hence, we shall find that not even the so-called false presentations (for example, those occurring in sleep or madness) are independent of things known to us through sense-perception by direct experience. . . . And in general one can find nothing in our conception which is not known to oneself in direct experience. For it is grasped either by similarity to what is revealed in direct experience or by expansion or reduction or compounding. (Against the Professors, 8.56-8)

We shall discuss thoughts that come "not without sense-perception" below.

According to the Stoic physics, sense-perceptions are the product of an interaction between the human soul and physical objects. Zeno called the result of this interaction an "impression," and it seems that he had in mind a literal stamping of the object on the soul (which, for the Stoics, is itself physical). Chrysippus amended this literal notion to accommodate the fact that one can have several impressions at once. Thus he used the more generic term "alteration" to describe the effect brought about in the soul by its interactions with objects. "For just as air, when many people speak at once, receiving at one time a number of different blows, also has many alterations, so too the leading part of the soul will experience something similar when it receives varied presentations" (Sextus Empiricus, Against the Professors, 7.231). Ultimately the description can be refined even further to refer to the part of the soul in which the alteration takes place, i.e., the "leading part," which is the seat of the intellect.


The Stoics recognized that it is not sense-perception alone which gives rise to knowledge or opinion. What must be added to the presence of a preception in the soul is "assent," which is a voluntary act. The problem of the veracity of sense-perception, then, lies in the possibility of giving assent wrongly. The Stoics postulated an idealized knower, the "wise man" or "sage," who never holds opinions. This end could be attained by withholding assent altogether, but the Stoics insisted that the wise man has knowledge, which means that his assent is always correct.

For the rest of us, it is easy to be deceived in cases of sense-perception. Deception, which always results in mere opinion, occurs when one assents wrongly to a perception. Thus the first task of the wise man is to be circumspect in giving assent to any impresssion. Ordinary people tend to assent to those perceptions which are forceful without giving the matter any further thought. Cicero describes Zeno as holding "that the wise man's chief strength is that he is careful not to be tricked and sees to it that he is not deceived" (Academica, 2.66 [III-9]). If he is successful, "he can distinguish falsehoods from truths and what is not perceptible from what is perceptible" (Academica, 2.67 [III-9]).

The "Graspable" Presentation

The key condition for this ability to distinguish falsehood from truth is that the sense-perceptions themselves must represent reality accurately. The Stoics called those which do so "graspable" presentations. To present the world as it really is, the presentation must first come from an object, rather than being a mere fantasy of the mind, "an experience in the soul which occurs as the result of no presented object, as in the case of people who fight with shadows and punch at thin air. For a presented object underlies the presentation, but no presented object [underlies] the 'phantastic'" (Aetius, 4.12.1). Secondly, the presentation must reveal the object which causes it to exist. As Cicero describes it, they "have a distinctive kind of clear statement to make about the objects of presentation" (Academica, 1.40. Another bit of testimony from Cicero is that "Zeno defined it as a presentation which came from an existing thing and which was formed, shaped and moulded exactly as that thing was" (Academica, 2.77 [III-10]). It is important to note that this conception fits best with Zeno's own definition of the impression as literally a stamp: "Just as seals on rings always stamp all their markings precisely on the wax, so those who have a cognition of objects should notice all their peculiarities" (Against the Professors, 7.5). It is not so clear that this can be said of a mere "alteration."

The Academic skeptics recognized a difficulty with Zeno's conception. Even if it is granted that a presentation shows a thing exactly as it is, there remains the possibility that other things could have this same quality, so it would be questionable which object is represented by such a presentation. Arcesilaus the skeptic is said by Cicero to have pressed Zeno on this point: "Could a true presentation be of the same quality of a false one?" (Academica, 2.77). Arcesilaus's presumption was that "what is false cannot be perceived and neither can what is true if it is just like what is false." Contemporary episemologists have made the same point: if one does not have the ability to distinguish perceptually between objects which are very similar (at least in their current environment), then even if they are correct in their assent, they lack knowledge. (See Alvin Goldman, "Discrimination and Perceptual Knowledge," The Journal of Philosophy 73, 2 (1976), 771-791.)

Sextus mentions a hypothetical case involving the twins Castor and Polydeuces. If the wise man gets an impression from Castor which looks just like one that would be had by perceiving Polydeuces, it will be "a false impression, albeit one from what is and imprinted and stamped exactly in accordance with what is." Another example Sextus cited was "two exactly similar eggs." Cicero reports that Arcesilaus tried to "show that there in fact existed no presentation coming from something true which was not such that one of the same quality could have come from something false" (Academica, 77). Even if it could be shown that there are no two things which resemble each other exactly, the point would still hold, since this degree of similarily

could certainly appear to exist and therefore deceive the sense, and if a single likeness has done that, it will have made everything doubtful. With that criterion removed which is the proper instrument of recognition, even if the man you are looking at is just the man you think you are looking at, you will not make the judgment with the mark you say you ought to, viz. one of a kind of which a false mark could not be. (Cicero, Academica, 2.84)

To defend themselves against this possibility, Sextus reports, the Stoics recognized that the graspable presentation must meet a further condition to serve as a criterion of truth: that it "has no impediment. This impression, being self-evident and striking, all but seizes us by the hair, they say, and pulls us to assent, needing nothing else to achieve this effect or to establish its difference from other impressions" (Against the Professors, 7.255). What Sextus has in mind as an "impediment" is something going wrong with respect to at least one of the five factors which are required for the presentation to be graspable: "the sense-organ, the sense-object, the place, the manner, and the mind" (Against the Professors, 7.424). Thus if the mind is in an abnormal state, or one twin is indistinguishable from another, there would be an impediment. The thinking behind this requirement is that the graspable impression is natural product of the interaction of objects and the soul in a normal environment. "For nature has given the sensory faculty and the impression which arises thereby as our light, as it were, for the recognition of the truth. So it is absurd to abrogate this kind of faculty and to rob ourselves of the light, so to speak" (Against the Professors, 7.259).

This reply would not satisfy a skeptic. The point about the general inability to tell the difference between very similar objects would still hold here, because the lack of an impediment is something that we cannot assume to be something we can discern. How can I tell that I am speaking to Polydeuces and not Castor, i.e., that there is no impediment on the part of the object? One answer might be that more investigation would reveal whether it was indeed the person I thought it was when my impression "pulled me to assent" that it was Polydeuces.

Whenever someone is keen to grasp something precisely, he is seen to chase after such an impression of his own accord, as when, in the case of visible things, he gets a dim impression of the object. He strains his sight and goes close to the visible object so as not to go wrong at all; he rubs his eyes and does just everything until he takes in a clear and striking impression of what he is judging, as though he thought the reliability of the cognition rested on this. (Sextus Empiricus Against the Professors, 7.255)
This kind of response still seems to fall short of a refutation of the skeptical position. No matter how many precautions one takes, and no matter how "clear and striking" the resulting presentation might be, there remains at least the possibility of error. Investigation may reduce the likelihood that an impediment has obstructed the production of a presentation which cannot but truly present its object, but it cannot eliminate all possible impediments. There seems to be a presupposition here that when one is in possession of a graspable presentation and there is no impediment, it is nature itself which forces assent to it. Even if we suppose that this is the case, it would seem that there may still be unsuspected impediments which might also force assent. At least, this possibility would have to be ruled out in order for one to have knowledge on the basis of possession of a graspable presentation.


Perhaps as a result of considerations of this kind, we find that the Stoics adopted an even tougher requirement for knowledge even than lacking an impediment. The graspable presentation is only a necessary condition for knowledge: one gains knowledge on the basis of the graspable presentation when it is "grasped in such a way that it could not be shaken by argument" (Cicero, Academica, 1.41). Only the wise man is capable of knowledge, unfortunately, "but they themselves are not in the habit of saying who is or has been wise" (Cicero, Academica, 2.145). Zeno is said to have summed up the situation with a metaphor.

When he held out his hand with open fingers, he would say, this is what a presentation is like." Then when he had closed his figers a bit, he said, "assent is like this." And when he had compressed it completely and made a fist, he said that this was grasping (and on the basis of this comparision he even gave it the name katalepsis [grasp], which had not previously existed). But when he put his left hand over it and compressed it tightly and powerfullly, he said that knowledge was this sort of thing, and that no one except the wise man possessed it. (Cicero, Academica, 2.145)

The Basic Grasp as Criterion

We are told that various Stoics proposed criteria of truth other than the graspable presentation. Most important among them are conceptions, which are generated ultimately from sense-perception. Augustine states that from the bodily senses "the soul acquired conceptions which they called ennoiai, i.e., those things which are clarified by definition; from this source (they said) the entire system of teaching and learning is generated and the links within it are forged" (The City of God, 8.7). Foremost among the conceptions is the basic grasp, "a natural conception of things which are universal" (Diogenes Laertius Lives of the Eminent Philosopohers, 7.54), which the Epicureans also claimed as a source of knowledge. What is immediately striking is that the Stoics and Epicureans disagreed sharply about the content of their allegedly natural conceptions, such as that of the gods, though they did agree that we have a basic grasp of their existence. Epictetus tried to overcome this objection by arguing that it is the application of basic grasps that causes disagreement.

Basic grasps are common to all men, and one basic grasp does not conflict with another; for which of us does not suppose that the good is advantageous and worth choosing and that one ought to go for it and pursue it in all circumstances? Which of us does not suppose that the good is honourable and fitting? So where does the conflict come from? In the application of basic grasps to individual substances; as when one man says, "He did well and is brave! [and someone else says,] "No, he is crazy." This is the source of conflict between men. The conflict between the Jews and Syrians and Egyptians and Romans is not about whether the sacred must be honored among all else and pursued in all instances, but about whether this particular thing, eating pork, is sacred or not. (Discourses, 1.22.1-4 [II-104])
It is not clear how much help this would be in resolving the conflict among conceptions of the gods, since they are individuals, not universals like goodness.

For the Stoic empiricism to achieve its ends, to generate a description of the most fundamental characteristics of the physical universe, it must be shown how basic grasps can be used to extend the information given in sense-perception. The Epicureans developed such an account, but no Stoic version has survived antiquity. We are given a clue in the passage from Augustine, that definition is involved, and the Stoics also made extensive use of analogical reasoning. Rather than attempt to develop an account of the Stoic epistemology regarding objects that cannot be presented to the sense, we shall instead make relevant comments on the arguments that were given by the Stoics to demonstrate the truth of their account of the physical world.

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