Philosophy 143 Lecture Notes

The Epistemology of Epicurus

Early Greek Epistemology

The Greek philosophers were on the whole very suspicious of of sense-perception as a source of knowledge. As already noted, Democritus, whose atomism formed the basis of Epicurus's physics, claimed that the senses yield only "obscure knowledge."

Obscure knowledge includes everything that is given by sight, hearing, smell, taste, touch, whereas genuine knowledge is something quite distinct from this. Whenever [an investigation reaches the point where] obscure knowledge can no longer see the objects because of their smallness, and also cannot hear or smell or taste them nor perceive them by touch, [the investigator must have recourse] to a finer means of knowing. (Wheelwright, The Presocratics, p. 183)
The problem with the senses is the "relativity of perception." Aristotle cites the contrariness of the sense-perceptions of different persons (the same food being thought sweet by some and bitter by others, for example), of animals other than ourselves, and even of the same person in different situations.
Which, then, of these impressions are true and which are false is not obvious; for the one set is no more true than the other, but both are alike. And this is why Democritus, at any rate, says that either there is no truth or to us at least it is not evident." (Metaphysics, Book IV, Chapter 5)
Interestingly, Aristotle concludes from this that thinkers, including Democritus, "say that what appears to our senses must be true," which was to become the view of Epicurus. If there is no means by which one set of sense-perceptions can be said to be truer than another, one might conclude that all are equally true, a view which Aristotle denounced vigorously. "Regarding the nature of truth, we must maintain that not everything which appears is true" (Metaphysics, Book IV, Chapter 5). The source of the error of those who say that all appearances are true is "that while they were inquiring into the truth of that which is, they thought 'that which is' was identical with the sensible world" (Metaphysics, Book IV, Chapter 5). Another philosopher who held this kind of view was the Sophist Protagoras, who drew from it a further consequence: "if all opinions and appearances are true, all statemnts must be at the same time true and false" (Metaphysics, Book IV, Chapter 5). Epicurus did not go this far.

The Canon

Epicurus's thoughts on the investigation of the true and the false are said to have been laid out in his book The Canon. The Greek term translated as 'canon' means 'ruler' or 'yardstick,' a measuring device. Epicurus's canon was gave him a way of measuring opinion (as well as appearance) with respect to truth or falsehood. This work has been lost, so our assessment of Epicurus's theory of knowledge must rely on other of his works, as well as works of his followers and opponents.

The Criteria of Truth

Given at least that some opinions are false, there arises a need to discover which are true and which are false. A criterion of truth is the instrument which allows us to distinguish knowledge from false opinion. According to Epicurus , there are three criteria of truth:

As we will see, this list is incomplete by itself and was in fact augmented by Epicurus's followers to include what links these fundamental criteria to opinion: (Diogenes Laertius, Lives of the Eminent Philosophers, 10.30).


According to Epicurus's atomic theory of the soul, sense-perception is completely passive. It is the result of the action of bodies (or the thin films thrown off by them) on the human sense-organs, and ultimately upon the soul. Thus Diogenes Laertius quotes Epicurus as claiming that "every sense-perception is unreasoning and incapable of remembering. For neither is it moved by itself nor can it add or subtract anything when moved by something else" (Lives of the Eminent Philosophers, 10.31) Because of its insulation from the reasoning part of the mind, sense-perception does not contain anything in it that could lead to error. Moreover, sense-perception cannot be refuted:

For a perception from one sense cannot refute another of the same type, because they are of equal strength; nor can a perception from one sense refute one from a different sense, because they do not judge the same object. Nor indeed can reasoning [refute them]; for all reasoning depends on the sense-perceptions. Nor can one sense-perception refute another, since we attend to them all." Lives of the Eminent Philosophers, 10.31-32)
Democritus had made a similar point: that any criticism of sense-perception by the intellect is undermines the authority of the intellect itself.
A dialogue between the intellect and the senses.

INTELLECT: It is by convention that color exists, by convention sweet, by convention bitter.

SENSES: Ah, wretched intellect, you get your evidence only as we give it to you, and yet you try to overthrow us. That overthrow will be your downfall. (Wheewright, The Presocratics, p. 183)

The doctrine that all information available to the intellect comes through the senses in known as empiricism. Diogenes quotes Epicurus as stating, "all ideas are formed from sense-perceptions by direct experience or by analogy or by similiarity or by compounding, with reasoning also making a contribution" (Lives of the Eminent Philosophers, 10.33). The contribution of sense-perception to the detection of truth, then, is that it is the only material whereby truth can be known. As we will see, Epicurus went further and maintained that sense-perceptions themselves are all true.

Basic Grasps

The second criterion is the prolêpsis or "basic grasp" (also translated as pre-conception). This is what serves as a "universal idea," which is based on "a memory of what has often appeared in the external world" (Lives of the Eminent Philosophers, 10.33). It is also the denotation of words, such as "man," which, when uttered, gives rise to "an idea of the general outline of man." The basic grasp is essential to human knowledge, since it provides the conception which we bring to our inquiry (hence the appropriateness of the term "pre-conception"): "we could never have inquired into an object if we had not first been aware of it," as when we ask "is what is standing far off a horse or a cow?" (Lives of the Eminent Philosophers, 10.33),

The doctrine of the basic grasp becomes complicated when Epicurus adds that every person has a basic grasp of the gods, which do not appear to us in the way that men, horses or cows do. One explanation of this pre-conception is that the soul receives images from the gods, who exist in the space between the various cosmoi. This, however, conflicts with the text which says that the images flow to the gods (though it is often translated as saying that the images flow from them). Long and Sedley try to solve this puzzle by maintaining that the gods themselves are no more than the products of thought, based on an ideal we form of a blessed human being.


The last of the original criteria is the feelings of pleasure and pain. These criteria allow us to judge which actions we should undertake, in that pleasure is to be sought and pain avoided. However, Epicurus also assigned them a role in knowledge as well, as in the "Letter to Heroditus," 64, where he appeals to feelings as a source of information about the makeup of the soul.

Presentation vs. Opinion

According to Sextus Empiricus, "Epicurus says that there are two things which are linked to each other, presentation and opinion, and that of these presentation, which he also calls 'clear fact,' is always true" (Against the Professors, 7.203). Epicurus himself contrasts opinion as "what awaits confirmation" with "what is already present in sense-perception, and the feelings, and every application of the intellect to presentation" (Principal Doctrines, XXIV). In the Letter to Heroditus, he distinguishes the criteria of truth from what is "non-evident."

Sextus adds that for Epicurus some opinions are true and some are false (Against the Professors 7.211). The reason is that we make judgments upon presentations and we judge some thiings correctly and some badly, either by adding and attaching something to the presentations or by subtracting something from them--in general terms, by falsifying the non-rational sense-perception." So opinion is true so long as the right things are added, etc. to the sense-perceptions, but the sense-perceptions are true in themselves.

"All Sense-Perceptions Are True"

We have already seen the germ of the radical doctrine that sense-perceptions are all true in the claim that the senses are entirely passive and cannot be refuted. This claim raises an immediate difficulty. Suppose the sense-perception in question involves the perception of something in front of me as being red. If I make the judgment "Something in front of me is red," it would seem that I am applying one or more basic grasps, e.g., the concept of red, in making the judgment. But in that case, it is not sense-perception by itself that is true, but rather the application of a basic grasp to a perception. It is for this general reason (though perhaps not as it arises in the case of simple perceptual claims) that the Epicureans added to the criterion "the applications of the intellect to presentations."

Supposing that it is judgments of the kind just given that are supposed to be true one and all, the next question is how they can be allowed to be true without violating the principle of non-contradiction (as Progagoras was willing to do). That is, if a piece of fruit tastes sweet to me and bitter to you, how could the judgments based on our sense-perceptions both be true?

A radical interpretation is to hold that Epicurus is not claiming that both judgments are true, but only that both sense-perceptions are real. This claim is based on the ambiguity of the Greek word (aletheia), which might be translated as "reality" rather than "truth." This interpretation, however, would not account for the debates between the Epicureans and their opponents regarding the consequences of the Epicurean claim, which would be uncontroversial if understood as affirming merely the reality of sense-perceptions. On the other hand, it must be noted that Epicurus responded to the criticism that a vision of the Furies is true by claiming that the person really has the vision. (Sextus Empiricus, Against the Professors, 8.63-4).

Another way to understand the claim that all sense-perceptions are true is by holding that the content of judgments of sense-perception are limited in their scope. Rather than judging that the fruit is sweet, I could merely be judging that it tastes sweet to me. This is attributed to the fact that different aspects of an object affect people in different ways

since everything is combined and blended together and since diferent things are designed by nature to fit into different [pores], it is not possible for everyone to touch and grasp the same quality; nor does the obejct [of sense perception] affect everyone the same way with all its parts, but all of them only experience those parts [of an object] with which their sense-organ is symmetrical; so they are wrong to quarrel about whether the object is good or bad or white or not white, supposing that they are supporting their own sense-perception, since all sense-perceptions make contact with something, each drawing what is compatible and suitable to itself from the compound mixture as though from a spring; and must not assert [things] about the whole when one is in contact with [mere] parts, nor think that everyone has the same experiences according to the differing qualities and powers of it. (Plutarch Against Colotes, 109d-e)
This point of view is ridiculed by Plutarch, on the grounds that such a judgment does not reveal anything about the external object. Addressing the Epicurean Colotes, he writes:
[Epicurus] sees what follows better than you do, and he sticks with it: viz. that every presentation on its own account is equally trustworthy and that no presentation is preferable to another, but that all are of equal value. But you are giving up the principle that all [perceptions] are true and that none is unreliable or false if you think that based on these one ought to further pronounce regarding external objects, but did not trust them for anything beyond the experience itself.

A further way of interpreting the claim that all sense-perceptions are true is based on the account of Sextus Empiricus, who holds that it is the atomic image which affects the sense organs that is the subject of perceptual judgments. For example, the image that reaches us from a distant tower is really small, and so vision

tells the truth, since when the object of perception appears to it [as] small and of such a shape it is genuinely small and of such a shape (for the edges of the images are broken off by the movement through the air), and when it again appears big and of a different shape, again it is in a similar manner big and has that different shape--the object being, however, not now the same in the two cases. For it remains for distorted opinion to think that the same object of presentation was observed from close up and from a distance." (Against the Professors 7.209)
The issue raised by this account is whether the truth about images tells us anything about the nature of the bodies which produce them, except perhaps those aspects of the bodies which relate directly to sense-perception. But classifying bodies under general concepts, as in the judgment, "That is a horse," or applying names to them, as in "That is Plato," seems vulnerable to being false, given that this judgment goes beyond the "experience itself." They would lie in the realm of opinion.


As was stated above, Sextus describes a difference between the "clear fact," which is always true, and opinion. These clear facts "are the foundation and cornerstone" of the truth and falsehood of opinion. The exact way in which clear facts related to truth and falsehood, as presented by Sextus, is somewhat confusing. It is also unclear exactly what the clear facts are, given the discussion just completed.

Therefore, according to Epicurus, some opinions are true and some are false; those which are testified for and those which are not testified against are true, while those which are testified against and those which are not testified for by clear facts are false. (Against the Professors, 7.211. Subsequent quotes are from sections 212-216.)
Let us begin with the pair, which Sextus says are in opposition to each other, "testified for" and "not testified for." In explaining how opinions which are testified for are true and those which are not testified for are false, he uses the case of a person approaching at a distance, such that it is not a "clear fact" that it is Plato. The opinion that it is Plato is true just in case on the person's near approach, "the clear facts themselves testified to it." Falsification of opinion occurs when there are clear facts to show that the opinion is untenable. In the case of the approaching Plato, "when the distance is reduced we realize through clear facts that it is not Plato. "

What is puzzling here is the terminology, where one would assume that the clear facts testify against the opinion that the approaching figure is Plato. What Epicurus seems to have had in mind is that when Plato is near at hand, the lack of clear facts that confirm that it is Plato is sufficient to confirm that it is not Plato. Obviously, then, "not testified for" can legitimately be used to falsify opinion only when one is in a position where there should be testimony for, but that testimony is lacking.

One must assume that the clear facts here are sense-perceptions that conform or do not conform to the basic grasp of Plato, so the criterion of truth is the application of the intellect to the perceptions. Sextus does not state at what point the testimony of the clear facts is adequate to verify or falsify opinion, and it is not at all clear that Epicurus had the resources to describe and justify any given threshhold of adequacy.

The second pair is "not testified against and testified against." Epicurus's example, the opinion that the void exists, suggests that it is intended to apply to cases where the clear facts cannot directly verify or falsify the opinion. The void is a "non-evident object" which does not fall within the scope of sense-perception. As Sextus had stated section 210, "It is a property of sense-perception to grasp only that which is present and stimulating it," which is something that the void by definition could not do.

Here is is best to start with the positive criterion "testified against." Sextus describes this as "the joint elimination of the apparent along with the supposed non-evident thing." Thus if one holds that there is no void (as the Stoics did), then they are eliminating the non-evident thing, the void. But eliminating the void also eliminates motion, which is something apparent (see the next paragraph). Thus the opinion that there is no void is false.

What verifies the true opinion that the void exists the fact that its existence does not testify against the clear fact of motion: "for if the void does not exist, then motion ought not to exist, since the moving body would have not place to shift into because everything [would] be full and dense; consequently, since there is motion what is apparent does not testify against the non-evident thing which is the object of opinion." As with the case of what is "not testified for," we must be careful to recognize that not being testified against can serve as a tool to judge opinion only when there is a situation where, if the opinion were false, it would be testified against.

This interpretation explains why Epicurus could not verify his meterological hypotheses that he outlines in his "Letter to Pythocles." If opinion about the non-evident could be shown to be true simply by being consistent with the phenomena, then when there are several competing hypotheses, they would all have to be said to be true.

[Our aim is] neither to achieve the impossible, even by force, nor to maintain a theory which is in all respects similar either to our discussions on the ways of life or to our clarification of other questions in physics, such as the thesis that the totality [of things] consists of bodies an intangible nature, and that the elements are atomic, and all such things as are consistent with the phenomena in only one way. This is not the case with meteorological phenomena, but rather these phenomena admit of several different explanations for their coming to be and several different accounts of their existence which are consistent with our sense-perceptions. (Diogenes Laertius, Lives of the Eminent Philosophers, 10.83-116. Emphasis is mine.)
This interpretation, if correct, has consequences for Epicurus's case for the void. This is based on the claim that if there were no void, it would be testified against by the senses, which reveal the reality of motion. But Aristotle had given a competing explanation of motion without a void, which would make his claim that there is no void "consistent with our sense-perceptions." Then the theory of the void would be on a par with the meteorological hypotheses, a result Epicurus was trying to avoid. He could do this only by discrediting Aristotle's account of motion within a plenum. But we have no evidence of such an effort.
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