The path of knowledge: the theaetetus by Robert Cavalier

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by Plato
(Written 360 B.C.E)

Translated by Benjamin Jowett


by Robert Cavalier

The Theaetetus can be considered a Socratic dialogue, since in it we do not arrive at any definitive answers to the questions which are posed. Its central concern is the problem of knowledge, yet its main conclusions all serve to show us what knowledge is not. Be this as it may, the Theaetetus rightfully belongs to the later set of dialogues since it prepares the way for the truly Platonic analyses of knowledge which are found in the Sophist. The Theaetetus, by clearing away many false opinions, allows Plato to introduce his own full-blown theory, a theory which connects the problem of knowledge with the realm of the Forms. Because of this interconnection between the two dialogues, and because the analyses of the Sophist presuppose the negative critiques of the Theaetetus, we shall begin our path of knowledge with the Socratic problem.

The dialogue opens with a brief prologue which serves to date the time of the supposed conversation. An introduction then guides the reader into the setting for the discussions which were to have taken place between an aging Socrates and a youthful Theaetetus. It is here that the dialogue is given its direction through the posing of its central question: "What is the nature of knowledge?"

Theaetetus makes three general attempts to answer this question, and his responses form the major divisions of the work. The first attempt tries to equate knowledge with sense perception; the second speaks of knowledge as true judgment (but how do we know that a judgment is true?); the third response augments the second by saying that knowledge is true judgment accompanied by an explanation. Yet Socrates is able to show Theaetetus that each attempt to arrive at an absolute answer to the problem of knowledge is fatally flawed. In the end, we are left with an awareness of our ignorance concerning the nature of knowledge (and the way is prepared for the more thoroughgoing analyses of the Sophist).

The Prologue (1-71)

Through the eyes of Eucleides, we see a sick and mortally, wounded Theaetetus being carried back to Athens from the battlefields near Corinth. The dialogue begins as Eucleides encounters Terpsion of Megara. Both are now middle-aged, and both were present at the death of Socrates some 35 years ago. Eucleldes tells Terpsion of the sad sight he has just seen, and recalls how Socrates, as an old man, had once had a very stimulating conversation with a then young Theaetetus. Eucleides has a copy of that dialogue and, at Terpsion's request, they retire to Eucleides' house where a servant boy is bought out to read the text. It is a fitting way to remember the wise and noble Theaetetus. . .

Introduction (72-528)

The scene opens with Socrates enquiring of the visiting geometer, Theodorus of Cyrene, if there were any young men in Athens who had impressed him. Theodorus responds by saying that there was a young man, very similar in appearance to Socrates himself, whose name was Theaetetus. At this moment, three well-oiled boys are seen walking down the street, and Theodorus points out Theaetetus as the one in the middle. He gestures to the youth to come and meet Socrates.

At first, Socrates compares their physical likeness, noting that both he and Theaetetus are short, stout, and snubnosed. The conversation, however. moves quickly from the similarity of their bodies to the similarity of their souls. Are they alike in intellect as well? To test this, Socrates asks Theaetetus to join him in solving a problem. The problem's general form concerns the relation of knowledge to wisdom. But before investigating the relationship of the two, one must have a clear idea of each. At present, Socrates is interested in the problem of knowledge. This, then, will form the central topic of the dialogue. Theaetetus' abilities will be put to the "test" through his attempts to answer the question: "What, precisely, is knowledge?" (217).

Theodorus, the man of mathematical figures, is not at home with a question like the problem of knowledge as such. He excuses himself from any active part in the conversation, thus leaving Theaetetus and Socrates to fend for themselves.

Theaetetus at first responds to Socrates' question by simply giving instances of knowledge: the things one learns in geometry, the things one can learn from a cobbler, and so forth. These examples of knowledge, Theaetetus believes, give us an answer to the question concerning the nature of knowledge.

But Socrates notes that this first answer does not so much address knowledge as it does the particular objects of knowledge. For instance, the things one learns in geometry are mathematical rules and figures, the things one learns in cobbling are leather-tanning and sewing. Yet the question was not "What are the objects of knowledge?" nor "How many kinds of knowledge are there?" but rather, and quite simply, "What is knowledge itself?" (247).

We want a "definition" of knowledge in and of itself, i.e., we want to grasp the nature or essence of knowledge. (We want, if you will, a knowledge of knowledge.) This, then, becomes the goal of the dialogue: To discover a "single character" that "runs through" all the particular instances of knowledge. It is only in this manner that we will arrive at an answer to the question posed.

Theaetetus is justifiably set back by such quick dialectical moves. He doubts that he is up to the task. Socrates, however, encourages the youth to continue. He points out that although he himself may be without any answers, he possesses a peculiar ability to help others in their search for wisdom. He then uses the image of a midwife to show Theaetetus what he means (395-526). (This is a key image, comparable to the image of Socrates as a gadfly. Both images are used by Plato to describe the nature of Socratic activity.) Like a midwife who is herself without child, Socrates goes about the town trying to help others give a successful birth (in his case, the birth of true knowledge). Again, like the midwife, he is capable of seeing i, the child to be brought forth is a phantom (dead and false) and, with this, he is also capable of determining a time for miscarriage (a dialectical end) if all is not going well.

This notion of intellectual midwifery sets the tone for the dialogue. Theaetetus, with the aid of Socrates' questions, will try to bring forth-from within himself--a correct idea of the nature of knowledge.

I. Knowledge is equated with sense perception (Aisthesis) (528-2402)

Since it certainly seems safe to say that one knows something when one is looking at it, or feeling it, or tasting it, etc., Theaetetus starts right off by saying that knowing is perceiving (530). Socrates exclaims that this seems like a good start, and he introduces the philosophies of Protagoras and Hereclitus to support Theaetetus' claim. Plato's strategy here is to investigate both philosophers with regard to a theory of perception, and to do this in such a way as to determine the truth of Theaetetus' opinion that all knowledge is essentially perception.

As will have been seen in the general introduction, Protagoras was considered a great Sophist. One of his principles was the doctrine that man is the measure of all things (i.e., everything is relative to an individual view). This doctrine applies to a theory of perception when we say that all immediate sensations are relative to the individual who is perceiving. For example, a certain container of water might feel cold to someone whose hands are warm and yet this same container might feel warm to someone whose hands are cold. In this case, each person would be infallibly correct about the sensation that is felt--and yet each would feel a different sensation. This is the meaning of the phrase "man is the measure" when applied to an individual's immediate sensation (528-588).

Socrates goes on to say that this Protagorean doctrine might contain a reference to another doctrine, a doctrine that comes from the philosopher Heraclitus (the thinker, remember, who said that "All things are in flux," and who represented the Principle of Becoming). With respect to "sensible reality" (i.e., the world as we perceive it), this Heraclitean doctrine would hold that all the things that we perceive are in motion (look at a flowing river, a swaying tree). Now Socrates wants to argue that this doctrine applies even to apparently stationary objects.

To understand this interpretation of the Heraclitean principle, we will have to discuss Plato's theory of the process of sensation. For Plato, the "object" of sensation is really the twin offspring of a subject's coming into active contact with the motion of a physical object. Sensation is the con- junction ("intercourse") of the active sense organ and the (slow or fast) movement of the physical object. This conjunction yields the momentary (and changing) sensation of, for instance, a particular whiteness (652-673).

Plato's theory of the process of sensation thus augments the Heraclitean doctrine to the degree that the objects perceived in the physical world are in a constant state of change due to both the objects themselves and the sense organs that interact with them. This theory is then joined to the Protagorean doctrine which allows each particular individual to be the infallible witness and final judge of the sensations which arise from the intercourse between subject and object. The result of this is a description of the world of sensation as a world that is constantly changing and entirely relative to the percipients.

Plato now draws out even more drastic conclusions. We say that wine tastes sweet to a person when he is well, but that it tastes bitter to him when he is ill. Now, if sensation is really a twin offspring, then we must say that the wine becomes sweet for, say, Socrates when he is well, and that it becomes sour when Socrates is ill--and, further, we must conclude that even Socrates himself changes when his condition changes from health to illness! Such, then, is the relativity of sensation and the concommitent infallibility of the one who is perceiving. Everything we perceive is true for every changing person at any particular moment (1020-1027). With this conclusion, both Socrates and Theaetetus (and, presumably, Plato himself) come to accept the Protagorean and Heraclitean doctrines as they pertain to the world of sense experience.

But both of these doctrines have a more extreme formulation. Both doctrines have the possibility of being interpreted to apply beyond the realm of mere sensation and into the areas of valuation. Socrates will rigorously reject the extension of these doctrines, and the next stage of the dialogue is devoted to a criticism of the extreme formulations. Plato will first state the positions and then critique them.

In the extreme Protagorean doctrine, "man is the measure of all things " applies not merely to sensations, but also to the areas of wisdom, the knowledge of right and wrong, the problems of the Good and the Beautiful, etc. In this radical formulation, man truly becomes the measure of all things. Thus, for example, the Good becomes "that which appears good to a certain person at a certain time." All values and, indeed, the objects of all important knowledge, become radically relative in this extended view of the Protagorean doctrine (1057-1103).

The extreme Heraclitean doctrine, "everything is in flux," when carried beyond the world of sensation, is said to apply to all possible realities. Everything, at all times and in all places, is said to be in a state of constant flux and Becoming. (But to admit this would be to deny the possibility of Permanence, the possibility of anything that might be universal and unchanging. In a word, the extension of this doctrine would deny the possibility of the Forms themselves.)

We may now begin to grasp the depth of the problem. The extension of the Protagorean and Heraclitean doctrines poses a grave threat to Plato's own theory. For Plato, the Forms are the ultimate realities which ground the possibility of true knowledge. These Forms are neither relative to the individual, nor do they change from time to time. The Form of Justice, for instance, is not relative to an individual person or a particular state (contra the Protagorean doctrine) nor does it vary from epoch to epoch (contra the Heraclitean doctrine).

This is the Platonic problem that forms the background for the current discussion. Socrates' implicit task in the critiques which follow is to safeguard Plato's position against any extreme proponents of Protagoras and Heraclitus.

(A) Critique of the extreme Protagorean doctrine. As we have seen, the extension of Protagoras' doctrine states that each individual person is not only the "measure" of his or her own particular sense experience, but also that each particular person is the judge of what is right or wrong, good or evil, etc. Each person, in other words, becomes the measure (criterion) of all thought. In the extension of this doctrine, thought--and truth itself- become radically relative.

But if this is so, Socrates wonders, then this Protagorean proposition (viz., that "truth is relative to each person") would seem to entail the proposition that "everything is true," and that proposition would yield the conclusion that "nothing is false." But if that were the case, then a view contradicting Protagoras' (e.g., "There are some things--the Forms--which are not relative") would also be true (1571-1595).

Furthermore, it is certainly the case that some people are wiser than others in certain situations: The advice of a doctor and the advice of a carpenter in matters of health are not equally "true." This is born out in reference to future states of affairs. Would not the physician (i.e., the expert) be in a better position to predict a certain medical outcome than the carpenter? But this is surely to say that one man is capable of being more knowledgeable than another, and not to say that both are equally the "measure of truth" (1082-1103) (1611-1622) (1890-1900).

(B) Critique of the extreme Heraclitean doctrine. If the doctrine of Heraclitus is extended to cover all possible reality, then the world of knowledge, Socrates argues, would lack "substance." We would not have the permanence and stability that allow even our very words to have meaning. Truth, knowledge, and language itself would be impossible if everything were constantly changing. For example, if the word "apple" were to mean fruit at one moment, water at another, and so forth, then we wouldn't be able to communicate, we wouldn't "know" what we were talking about. Indeed, if everything, including words, were constantly changing, then we couldn't even formulate something like a "doctrine of the flux" (2140-2150). Socrates concludes that all cannot be in flux. (And Plato thus implies that there must be permanence somewhere. But this would take us into a discussion of Parmenides and the realm of the Forms, a discussion that is postponed until the Sophist.)

At this point, we can draw the conclusions of Socrates' dialectic with Protagoras and Heraclitus. Plato will accept the Protagorean doctrine that "man is the measure...," but only in so far as that doctrine pertains to sensation. He rejects its extension into the realm of truth and wisdom (i.e., the highest forms of knowledge). Plato will likewise accept the Heraclitean doctrine of the flux, but only in so far as it accords with his own theory of the process of sensation (and thus only in so far as it pertains to sensible reality). He rejects its extension into all possible reality (and thus, by anticipation, he rejects its extension into the realm of the Forms).

The dialectic with Protagoras and Heraclitus has established both the applicability of their doctrines and the limitation of their doctrines. We see, for instance, that their doctrines help us to understand the role that sensation plays in Plato's philosophy, and we see that this role is very limited. Sense perception may yield immediately certain and absolutely relative awareness of one's present condition, but it does not yield any important kind of knowledge. Indeed, sensations constitute only a very small part of what we properly call knowledge. For example, notions such as "existence" or "non-existence," "like" or "dislike," "Good" or "Evil," and many other instances, are not the kinds of things that are arrived at through the senses (2309-2387). Furthermore, since sensations can only yield the infallible evidence of a changing and momentary experience, they lack the ability to grasp the truth of things. That is to say, sense impressions do not contain within themselves the "nature"of things: What something "is" is ultimately grasped by reflection and thought (2352-2355).

This last consideration, however, casts into doubt the possibility that sense perception is at all capable of yielding any knowledge--for, if a man cannot arrive at the truth of a thing, then it cannot be said that he knows it (2331-2333). Knowledge does not reside in our sense impressions, but rather in our reflections upon those impressions. Consequently, perception (aisthesis) and knowledge (episteme) cannot be the same, and with this, Theaetetus' first attempt at a definition of knowledge is thoroughly refuted.

II. Knowledge is true judgment (doxa) (2402-3308)

The critique of perception has indicated that knowledge requires much more than mere sensation. It is not so much what affects the senses as what goes on in the mind; perhaps, then, it is not so much perception as judgment that yields knowledge. Theaetetus thus suggests that knowledge involves judgment and, more specifically, that knowledge could be defined as true judgment (2405).

Socrates takes Theaetetus' direction, but immediately poses the problem in terms of the phenomenon of false judgment (2429-2431). Theaetetus agrees to digress with Socrates, and both now set out to explain the possibility of making a false judgment (with the intention of then retrieving the notion of true judgment).

The problem centers around the problem of either knowing something or not knowing something (2450-2470). How can you not-know something you know (and thus make a false judgment)? On the other hand, if you do not know something, then the matter stops there--you cannot also know it (and then mistake it) (2463). For instance, if Socrates knows Theaetetus (as he does now), then he cannot "not know" him, and, if he doesn't know Theaetetus (as was the case before), then he can't know him (before he knows him). So here Socrates either knows Theaetetus or he doesn't. In this case, there seems to be no room for mistake. A false judgment seems impossible. Furthermore, there's another problem, and this problem revolves around the Sophist's theory about thinking that which "is not." Suppose that one were to define false judgment as thinking of something that is not (e.g., the judgment that there is a table in a certain room when in fact there is no table in the room). Certain Sophists would reply that such a judgment is impossible since, if one were thinking of something that is not, one would in effect be thinking nothing, and that is the same as not thinking at all (2550). But, if one is not thinking about anything, then one cannot be mistaken about anything! Finally, false judgment cannot be mistaking one thing for another (as when one mistakes "a horse for an ox") for, when one knows both, and when one is thinking of both at the same time, one simply can't think of one as the other, i.e., one can't be mistaken (2641-2672).

These admittedly convoluted arguments seem to revolve around the assumption that one must either be directly acquainted with the objects known (and these objects must be clearly present to the mind) or totally ignorant of them (i.e., these objects must be completely absent from the mind). But perhaps there is a way between these two alternatives, for surely false judgment must be possible. At this point, Socrates introduces the notion of memory. Memory contains a temporal aspect which allows for mistakes to occur even when one was previously acquainted with an object of experience. We are given two general descriptions of memory: Memory conceived of as a wax tablet and memory conceived of as an aviary.

(A) Memory as a wax tablet. Let us imagine, for the sake of argument, that the mind contains a tablet made of wax. The size and consistency of this wax block vary from individual to individual (i.e., some people have a better "memory" than others). And, like a wax tablet, the mind receives "impressions" in varying degrees of intensity. Some of these impressions, like childhood traumas, remain for a long time. Other impressions, like yesterday's morning shower, however, are weak and soon become forgotten (2718-2731).

Given this image of the memory, Socrates now goes on to try to explain the possibility of making a mistake (false judgment). One possibility occurs when a person sees two objects at a distance and, while being familiar with both (e.g., Theodorus seeing Socrates and Theaetetus) mistakes one for the other. In this instance, one fails to assign the appropriate sense impressions to the object, like thrusting a foot into the wrong shoe (2835). A similar possibility occurs when one has had sense impressions of both objects but presently perceives only one object and mistakes it for the other (e.g., Theodorus, seeing a round and short Socrates at a distance, mistakes him for the round and short Theaetetus) (2858).

This account of memory finally allows us to see the possibility of error. It gives Socrates a concrete example whereby he can refute the belief that false judgment is impossible. But it is likewise admitted that the examples are trivial and, further, that they cannot account for the more important judgments that are of a non-empirical nature (i.e., mistakes in the areas of mathematics and valuation). In order to expand the problem of false judgment, Socrates gives us another image of memory.

(B) Memory as an aviary. Let us imagine the mind as being initially an empty receptacle, and let's imagine further that it gradually fills up and becomes crowded with bits of knowledge. These bits of knowledge can be likened to birds flying about in an aviary (3075-3087). This image of the aviary, first of all, allows one to "possess" knowledge of either sense impressions or non-empirical beliefs. Furthermore, it allows us to recall these bits of knowledge without always having to refer to direct sensations.

Given this expanded image of memory, we may now suppose that, in making a false judgment, one "reaches into the aviary" for some "piece of knowledge" that is possessed and flying about, and one mistakenly grasps the wrong piece of knowledge. For example, in seeking the answer to the addition of 5 and 7, one "grasps" the number 11 instead of 12. This would seem to be an example of a (non-empirical) false judgment. (And to grasp the number 12 would be an example of a true judgment.)

But Socrates immediately feels uncomfortable with this solution. Taken as a whole, how could one have a knowledge of, say, 5 plus 7 equaling 12, and yet at the same time fail to recognize that very thing (and believe that 5 plus 7 equals 11) (3160-3201)?

Theaetetus suggests that one may have "pieces of ignorance" flying about with the "pieces of knowledge," and that error arises when we grasp a piece of ignorance (3203).

But Socrates points out that this answer only serves to heighten the basic problem (the basic circularity): How am I to know that I know? i.e., how am I to recognize and distinguish a piece of knowledge from a piece of ignorance? This seems to be the crux of the matter (3210-3251).

At bottom, we have not gotten any closer to a solution to the problem of knowledge. The matter of false judgment and the images of memory have only heightened the stalemate: How do I know that my judgment (my opinion) is true? There is no criterion yet for distinguishing true from false judgment. (And we will not find this criterion until we find the Forms, the final arbiter of truth.)

As long as we stay within the confines of the present argument (an argument that has no appeal to the realm of the Forms), we can at best have only true belief (e.g., I believe that "5 + 7 = 12," or that "justice is good," etc.). But even this is only accidental, for within the image of the aviary, when I recall a piece of knowledge, my attitude toward it is indistinguishable from my attitude toward a false belief. Different people will hold on to opposing beliefs with great tenacity, they will even kill one another for what they feel to be true beliefs. But this, for Plato, only shows the important distinction between true belief (doxa) and knowledge (episteme). (The latter is grounded in the universality of the Forms -- which are, in turn, grasped by the intellect in an unerring way.) True belief is too similar to mere opinion. The notion of judgment must be further buttressed if we are going to pursue it as an avenue to knowledge....

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