Theaetetus is one of the finest of Plato’s middle-period dialogues. It may well have been written as a tribute to the historical Theaetetus shortly after Theaetetus’s death from wounds suffered in battle. This conjecture rests on the fact that the speakers who introduce the main dialogue, but play no other role, refer to the return of the dying Theaetetus. This event serves as an occasion for them to read together a report of a conversation that took place a number of years previously between Socrates and Theaetetus. At the time, Socrates was awaiting his trial, so we are to assume that Socrates was seventy years old and that Theaetetus was a youth of about sixteen.

The dialogue proper opens with a conversation between Theodorus (Theaetetus’s teacher) and Socrates in which Theodorus praises Theaetetus highly. Socrates is impressed, and he calls the boy over to converse with him to see if Theodorus’s estimate is a fair one. Socrates tells Theaetetus of his occupation as an intellectual midwife and requests Theaetetus to let him use his art to see if Theaetetus will give birth to anything. The boy responds eagerly but respectfully, and the philosophical portion of the dialogue gets under way.

Knowledge as Perception

Socrates asks Theaetetus for a definition of knowledge, and Theaetetus replies that knowledge is perception. This possibility is then examined. Socrates, using his customary question-and-answer technique, proceeds to make the definition more precise. He begins by identifying the theory as that of Protagoras. With this identification, the theory is recognized as the familiar Protagorean view that humanity is the measure of all things. What the view comes to, Socrates states, is an identification of appearance with reality: “What seems or appears to me is to me.” The world of knowledge is in some sense private to each knower. The theory is thus applied to sensation, which is interpreted as an interaction or resultant of two elements, a sense stimulus and the sensory response. The stimulus is given status as something objectively real. Nevertheless, because each knower’s sense organs are private and the knowledge one obtains is conditioned by this private character, the result is a private world of knowledge.

Socrates wishes to clarify the theory still more, however. To do so, he points out that certain puzzles can arise if the theory is not fully understood. There are three “laws” that seem to be true, yet incompatible with experience:1. No object can become greater or smaller without having something added to or subtracted from it. 2. No object to which nothing is either added or subtracted is made greater or smaller. 3. Any object that now is, but previously was not, must have suffered becoming.

Against the background of these three apparently self-evident laws, it seems impossible that Socrates can now be taller than Theaetetus but yet within a year be shorter than Theaetetus, unless Socrates himself undergoes some change in height.

The resolution of the apparent conflict between the facts (Socrates’ first being taller, then shorter, than Theaetetus) and the three laws rests on recognizing that the...

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Redefining Knowledge

From this criticism of the extreme Heraclitean position, Socrates moves on to a total rejection of the definition of knowledge that Theaetetus initially put forth; namely, that knowledge is perception. At the beginning of the examination of this theory, Socrates laid down two criteria for knowledge that now serve as a basis for the rejection of Theaetetus’s first proposal. Socrates at that point stated—and Theaetetus had assented—that knowledge was of the real and infallible. The examination of the theory that knowledge is perception has issued in the recognition that sensation may very well yield something that is infallible, but it has also revealed that it is not of the real because the real is fixed and unchanging, not an ultimate flux. Yet Socrates goes even further in refuting Theaetetus’s initial proposal. He introduces a consideration that had been hinted at earlier but was then left undeveloped; namely, that knowledge involves a mind that interprets the deliverances of sense.

Before anything even remotely resembling knowledge is achieved, the raw data of sense must be interpreted. One may taste an apple, but one cannot judge that it is sweet without evaluating the sense experience in the light of a standard that is not in itself part of the raw taste of the apple; one must know what sweetness is before one can determine that the apple tastes sweet. This standard—which is the idea sweetness, the reader realizes, even though Plato avoids...

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True and False Opinion

Theaetetus’s first attempt to define knowledge has thus proved to be inadequate in the light of Socrates’ examination of it. If the dialogue is to continue, Theaetetus must offer a new definition. This he does, proposing now that knowledge is true opinion. The examination of this possibility occupies Socrates and Theaetetus in the next portion of the dialogue. However, if there is any significance to the definition of knowledge as “true opinion,” there must be something that is false opinion. It is to the elucidation of mistaken belief, or false opinion, then, that Socrates turns at this point.

Two well-known analogies occur in the account of false belief: the wax tablet analogy and the birdcage analogy. Socrates first suggests that belief may be analogous to fitting a new sense experience into an impression left in a wax tablet by a previous experience. When the new sense experience matches the impression in the wax, there is a case of true belief, but when the new experience is fitted into an impression that does not match, the result is mistaken belief.

The birdcage analogy takes into account some complications that the wax tablet analogy overlooks. The wax tablet analogy cannot account for mistaken belief about matters that do not have reference to sense experience. For example, a person might believe that seven plus five equals eleven, a mathematical belief that does not rest on sense experience. This situation is likened to a person who has a cage full of birds. At some time in the past, this...

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A Negative Conclusion

The conclusion of the dialogue, then, is that knowledge is neither sensation nor true opinion—not even true opinion accompanied by an explanation. Theaetetus has labored, but he has not brought forth a legitimate intellectual offspring. Nevertheless, Socrates says, the discussion has been of value to Theaetetus, for he does not now think that he knows something that he does not know, and if he attempts to define knowledge again at a later time his present efforts will help him to avoid certain pitfalls.

The overt conclusion of the dialogue may be negative, but for one who reads Plato intelligently and enters into the dialogue as an attentive, though silent, participant, the positive conclusion is nevertheless obvious, even though it is not explicitly stated: Knowledge can only be of the forms or ideas. To give an adequate account of knowing one must introduce the ideas. This task Plato postpones until the Sophist, the dialogue that naturally follows Theaetetus.

One can hardly read Theaetetus without feeling that here is philosophy unsurpassed. Plato’s problems, for the most part, retain their vitality, and his solutions retain their interest, even after more than two thousand years. When philosophical excellence of the very first order is given expression in literary and dramatic form of equally high quality, there is very little that can be said in criticism. Part of Plato’s appeal undoubtedly results from the fact that he usually is occupied in rejecting inadequate positions, leaving his own positive doctrine to be worked out by the reader as the implicit alternative. It is certainly true that Plato made mistakes. Yet when his mistakes are all pointed out, and an allowance is made for his indirect manner of arguing for his own doctrine, the fact remains that the Platonic position is still in its essentials the same as that Plato himself held; the necessary corrections are minor. Platonism is a live alternative. The student who wishes to see the inadequacies of a simple-headed empiricism that thinks it can do without a judging mind in accounting for knowledge can do no better than to study carefully Plato’s Theaetetus.


Additional Reading

Brumbaugh, Robert S. Plato for the Modern Age. New York: Crowell-Collier, 1962. A good introduction to Plato’s thought and the Greek world in which he developed it.

Copleston, Frederick. A History of Philosophy: Greece and Rome. Garden City, N.Y.: Doubleday, 1969. Copleston devotes several clear chapters to a discussion of the full range of Plato’s view.

Cropsey, Joseph. Plato’s World: Man’s Place in the Cosmos. Chicago: University of Chicago Press, 1995. Discusses Plato’s views on human nature with attention to his political theories.


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