Version 2.0, October 19, 2005
Immediately following the death of Aristotle, there arose two new philosophical movements to rival the schools of Plato (The Academy) and Aristotle (The Lyceum). One was led by Epicurus. The Epicurean theory of knowledge holds little interest today, partly because of the implausibility of its main thesis, that all perceptual judgments are true. Epicurus had something to say about way in which testimony functions as evidence, but this doctrine is hard to understand and has had minimal historical influence. For these reasons, we turn our attention to the other rival school.
The epistemology of the Stoics is of interest largely because it involves some approaches that are replicated in more recent treatments of knowledge. Of particular interest are disputes which raged between the Stoics and members of Plato's Academy.
The leading Stoics were the founder Zeno, his disciple Cleanthes, and the most profound Stoic epistemologist, Chrysippus. Unfortunately, the information we have about the Stoics comes from commentators who lived much later, some of whom were quite hostile to the Stoic program. For an excellent account of Stoicism, including a discussion of their epistemology (under the title "Stoic Logic"), see the Stanford Encyclopedia of Philosophy entry, by Professor Dirk Baltzy.
The Origin of Knowledge in the Senses
The Stoic epistemology begins with the descriptive project. The Stoic physics describes the character of the universe, including that of human beings. One aspect of human nature is that t 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. The following account of the origin of human thought is attributed to Aetius, who lived in the first century C.E.
When a man is born, the Stoics say, he has the commanding-part of his soul like a sheet of paper ready for writing upon. On this he inscribes each one of his conceptions. The first method of inscription is through the senses. (4.11.1-4)The image of blank paper would be resurrected by John Locke in the seventeenth century. Both Locke and the Stoics were what we now call "empiricists," in opposition to the Platonic rationalism. The chief tenet of empiricism is that sense-perception is the starting-point of all knowledge.
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 a soul can bear 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).
The Stoics recognized that it is not the mere presence of sense-impressions, or of intellectual presentations derived from them, which gives rise to knowledge, or to opinion, for that matter. At the very least, what must be added to the presence of a perception in the soul is "assent," which is the taking of the presentation to be true. In a metaphor to which we will return, Zeno compared an impression to an open palm, while assent is like a hand with its fingers somewhat clenched.
For the Stoics, knowledge is infallible, while opinion is fallible. We may begin to build an anlaysis of knowledge, based on what has just been said, by laying down the following minimal conditions:
If S knows that P, then (1) S has a presentation that P and (2) S assents to the presentation.Note that both of these conditions are descriptive and not normative.
How does assent arise? A central tenet of the Stoic epistemology was that it is the character of the presentation itself that gives rise to assent. For example, if I have a visual impression that resembles Dion, then I am inclined to assent to the proposition that Dion is before me. Although natural, this inclination to assent in the presence of the appropriate presentations may be blocked under certain circumstances.
The early Stoics recognized that assent may be blocked as a result of madness. (And hence, madness blocks knowledge when it blocks assent.) The later Stoics recognized another condition that would block assent, which they called an "impediment." An impediment is something else to which one assents which undermines the truth of the current presentation.
For example, there is the story that Menelaus believed that he had left his wife Helen of Troy locked in his ship, and moreover that there had been many supernatural false apparitions of Helen. When presented with the real Helen, he did not assent to the proposition that his wife stood before him. So Menelaus did not know that he was confronting his wife due to his lack of assent.
Some presentations, however, are immune to any obstacles. Such a presentation, "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). These presentations will turn out to be central to the Stoic account of knowledge.
There are techniques available for us to induce assent when the presentation is not so striking as to do so at first. When someone has a dim presentation of a visible object,
he intensifies his gaze and draws close to the object of sight so as not to go wholly astray, and rubs his eyes and in general uses every means until he can receive a clear and striking presentation of that thing under inspection, as though he considered that the credibility of the apprehension depended on that. (Against the Professors, 7.258It makes no sense for the person to refrain from asserting even under these conditions.
Nature having kindled as it were a light for us, to aid in the discernment of truth, in the faculty of sense and the presentation which takes place by means thereof. it is absurd, then, to set aside so great a faculty and to rob ourselves as it were of our own daylight. (Against the Professors, 7.59-60).
We can see here that the Stoics were, in a sense, naturalists. They showed how the natural perceptual and intellectual faculties of human beings are adequate to satsify at least some necessary conditions of knowledge, and they were most concerned with describing how knowledge is attained. But they were forced by the Academic skeptics into the pursuit of the normative and validation projects.
If people assent only to those presentations which are true, then the necessary conditions for knowledge just laid down would also be sufficient. However, ordinary people frequently are induced to assent to false presentations. In such cases, they have mere opinion, (doxa), which can be described as "weak and false assent" (Sextus Empiricus, Against the Professors, 7.151.)
So a third condition for knowledge is needed.
If S knows that P, then (1) S has a presentation that P and (2) S assents to the presentation that P, and (3) the presentation that P is true.But adding this condition still does not yield an adequate analysis of knowledge. The Stoics made the further demand that in order to know that P, S be able to distinguish true presentations from false presentations.
If S knows that P, then (1) S has a presentation that P, (2) S assents to the presentation that P, and (3) the presentation that P is true, and (4) S is able to distinguish true presentations (of the same type as P) from false presentations.The ability to discriminate between true and false presentations is the mark of wisdom, and so it is the wise man, and the wise man alone, who is capable of knowledge. 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 skeptical commentator Sextus Empricus describes at least two potential reasons we may not be able to discriminate between truth and falsehood in our presentations. First, our presentations may be "vacuous," representing nothing, as in the case of deception by supernatural beings. Second, our presentations may be "distorted," as in the case of the mad Heracles, who could not recognize his own children, but thought them to be the children of Eurystheus (Against the Professors, 8.67). What is needed to complete the Stoic account of knowledge is to show how condition (4) can hold. The task is to describe some means (a "criterion") by which true presentations may be distinguished from false presentations.
The Stoics held that the criterion lies in the presentations themselves. Their basic claim was that the presentation whose truth can be determined must (in some sense) necessarily reflect reality accurately. The Stoics called those perceptions which do so "graspable." The "graspable presentation" (katalēpsis) "is one which is true and of such a kind that it could not turn out to be false" (Sextus Empiricus, Against the Professors, 7.152). It is the stepping-stone between true assent and knowledge. Knowledge is the domain of the wise and ignorance that of those lacking in wisdom, but the graspable presentation is available to both. We shall see later what it takes for someone assenting to the graspable presentation to attain knowledge.
The general characterization of the graspable presentation is expanded to encompass several conditions.
The first condition is designed to rule out the first kind of assent to a false presentation: "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).
The second condition rules out the second source of error cited above. Even if a presentation comes from an object, it may represent it inaccurately. If so, then it is not an adequate basis for knowledge. As Cicero describes it, the kind of perceptions required for knowledge "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, which embodies the first two conditions on graspable presentations. 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 to the subject which object is represented by such a presentation. Thus for a presentation to be a criterion of truth, it is required that no other object could make an impression that is indistinguishable from the graspable impression.
Arcesilaus the Academic 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 epistemologists 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." The first egg might make an impression of itself just as it is, but that impression could be mistaken for an impression of the other egg. Then the third condition would not be satisfied.
The Stoic defense against this kind of skeptical hypothesis is to make the following claim. If the perception of the object makes an impression which presents it in all its detail, then one will be able to distinguish it from all other objects, so long as there is some perceptible difference between them. The Academic skeptics did not concede this metaphysical claim. 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).
One way the Stoics tried to counter Arcesilaus's claim is to hold that there are no two perceptible things which resemble each other exactly. (Note that this does not follow from the stronger claim that no two objects have all their features in common, what is sometimes called "Leibniz's Law" or "the identity of indiscernibles." Unless it is held that all the features of an object are perceptible, it remains possible that the only feature with respect to which two objects differ is not perceptible.)
This claim would be difficult to back up, especially given the somewhat crude discriminatory ability of the human senses. This seems to have been the point of the skeptical rebuttal. Suppose you grant that all things are different in some way.
You say that such a degree of similarity does not exist in things . . . we will allow that for sure. Yet it can 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)We are told by Cicero that the question of infallible discrimination "is the one controversial issue which has lasted up to the present" (Cicero Academica, 2.78).
The Final Account of Knowledge
Perhaps as a result of considerations of this kind, we find that the Stoic criterion is not after all to be located in the graspable presentation by itself. What is required for the use of a graspable presentation as a criterion is that it be "grasped in such a way that it could not be shaken by argument" (Cicero, Academica, 1.41). Presumably, the argument in question is of the kind posed by the skeptics. The wise man, and the wise man alone, is capable of grasping his presentations in this way. Unfortunately, "but they themselves are not in the habit of saying who is or has been wise" (Cicero, Academica, 2.145).
We may now complete Zeno's 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 fingers 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 comparison he even gave it the name katalēpsis [grasp], which had not previously existed). But when he put his left hand over it and compressed it tightly and powerfully, he said that knowledge was this sort of thing, and that no one except the wise man possessed it. (Cicero, Academica, 2.145)At this point, the Stoics have set the standard as high as it can be set. The skeptic seems to have the advantage here. It is difficulty to see how any presentation of sense can be grasped in such a way that it can be unshaken by any argument. Some 2000 years later, Descartes would convince a lot of people that such a presentation cannot be found.
The Stoics as Externalists
It has been suggested that the Stoics would not be affected by the skeptical difficulties posed by Arcesilaus if they were externalists. The fundamental skeptical problem was that even if a presentation meets the three conditions of being graspable, we may not be able to tell that a presentation we have is in fact graspable. Only the wise man is in a position to determine that a given presentation is of the right sort.
An externalist reading would allow that merely being able to distinguish true from false presentations is sufficient for knowledge. The knower would not have to have an account of how it is done. We are told by Cicero that Zeno "attached reliablity to the senses," in part because "nature had given [cognition effected by them] as the standard of scientific knowledge and as the natural foundation for the subsequent impressions of conceptions of things upon the mind" (Academica 1.4.2).
If the Stoics were externalists, then they must have identified knowledge with assent to graspable presentations. This, in turn, would require that assent to such presentations is a sufficient condition for knowledge (epistēme.
It would then have to be "secure and firm and unchangeable by reason" (Sextus Empiricus, Against the Professors, 7.151). The point of the skeptical examples is that assent even to a graspable presentation is insecure and changeable by reason. I might have a graspable presentation of a man yet be unable to distinguish him from a nearby identical twin. So again, given the Stoic requirement that knowledge be infallible, it looks as if only the wise man is capable of knowledge, given the high standard of infallibility, even if ordinary people are in the possession of graspable presentations.
A final point about the Stoics is that they might be able to save their position by severely weakening the scope of knowledge, restricting it to appearances. We are told that the Stoic philosopher Sphaerus was once fooled into assenting that an object was a pomegranate, when in fact it was made of wax. His response was that he had only assented to the claim that it appeared to be a pomegranate. This has since come to be recognized as a standard move against the skeptic. But however secure such appearance-claims may be, they are not the sort of assent in which the Stoics were interested, and it does not require a wise person to make them.
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