a suggestion that size is an absolute rather than a relational property, that Socrates is short because of the presence of shortness (the idea) in Socrates. In such a view, the comparison of Socrates and Theaetetus would involve one in saying that initially tallness was present in Socrates, and then, a year later (without Socrates having undergone any change), shortness is present in Socrates. If sensation yields knowledge of the real, and Socrates becomes shorter than Theaetetus in virtue of shortness replacing tallness, one cannot say that Socrates really remains unchanged—yet Socrates does remain the same height. The contradiction lies in saying that Socrates changes and that Socrates does not change.

The account of sensation developed in this dialogue, however, is that the sensation of Socrates’ height is the result of an interaction between the sense stimulus and the sense organ. This is a more sophisticated account that, it should be noted, is intended to deal with sense experience, not with knowledge of the ideas. The ideas cannot be known by sensation, so a part of the problem—really knowing the tallness in Socrates—is dispelled. What can be sensed is the world of becoming, and this, for Plato and Socrates, is a world of flux, not the world that truly can be known.

However, the sensation—that which can be “known” on the basis of the theory under consideration—is a product of the interaction of an element from the flux with the private sense organ of an individual knower. The externally real combines with a private sense organ to give rise to sensation. The real, as Protagoras understood it, may very well be a collection of apparently contradictory qualities, but the world of “knowledge” is private because it is conditioned by the privacy of the knower’s sense organs. All sense knowledge is relative to a particular knower at a particular time. Thus, there is no contradiction in saying that Socrates is taller than Theaetetus, but later is shorter than Theaetetus. There is no stable, unchanging background to give rise to the (apparent) contradictions. The “laws” are thus revealed as resting on the conviction that there is an abiding, changeless structure to the world of sensation. Once this abiding character is rejected in favor of a flux, the “laws” lose their point. The external world is a flux, the knower himself is in flux, and sensation is a product of the two. If either changes, there is a different sensation.

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 specific mention of the ideas in this dialogue—must be unchanging and real. It can be known, and it serves as the standard in terms of which the raw data of sense are interpreted. However, the sense organs by themselves cannot know the standard. On the contrary, it is the mind that knows the standard and judges that the sense experience is of such and such a character in the light of this standard.

Uninterpreted sense experience, therefore, cannot possibly be knowledge; even if it is interpreted, it still has as its object something that is less than real, and the interpretation presupposes a standard that is the genuine object of knowledge. When pursued far enough, the examination of sense experience leads to the Platonic position that knowledge must be restricted to the ideas; we cannot have genuine knowledge of the world of becoming. Plato reserves the discussion of the ideas in their own right for Sophists (later period, 365-361 b.c.e.; Sophist, 1804), but Theaetetus prepares the ground for this later direct analysis of knowing.

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 individual got possession of the birds (learned the truths of mathematics) and put them into the cage. Later, he reaches into the cage to get possession of a particular bird. He may think he has grasped a parrot, even though he has a pigeon in his hand. Analogously, a person who once learned that seven plus five equals twelve may, when he tries to recall this truth, mistakenly believe that seven plus five equals eleven. He has in his possession, but not in his hand, the truth; thus, he describes incorrectly what he has in his hand.

These two accounts have a common feature. Both involve the interpretation of some conceptual object. It is the interpretation that brings in the difficulty, for whether one mistakenly judges that a stone is an apple (sense object) or whether one mistakenly judges that seven plus five equals eleven (mathematical object), one still is making an interpretation. Yet if the interpretation yields genuine knowledge, it must be infallible and of the real. A mistaken belief is clearly not infallible. If one knows and knows that one knows, one’s initial judgment cannot be fallible. Neither the wax tablet analogy nor the birdcage analogy yields such certainty, however, and they therefore fail to do the job. Plato and Socrates want a psychological criterion for separating true from false beliefs. Neither the wax tablet nor the birdcage yields such a psychological criterion. Both accounts of mistaken belief are therefore inadequate, but the discussion is broken off at this point since further analysis would require Plato to introduce the ideas, something he wishes to postpone until the Sophist.

Theaetetus has one final proposal to make. He suggests that the difficulty encountered in the examination of false belief may be overcome by defining knowledge as true belief accompanied by a reason or correct explanation. This will indeed do the job, but it has one crucial flaw; namely, it is a circular definition of knowledge. What the definition comes to is that knowledge is true opinion that is known to be true. An adequate definition, however, cannot use the term to be defined in the definition of the term. Theaetetus’s final suggestion is inadequate, just as his earlier proposals have been.

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.