Returning then to the Theaetetus, as the only possible source from which an answer to these questions can be obtained, we may remark, that Plato had 'The Truth' of Protagoras before him, and frequently refers to the book. He seems to say expressly, that in this work the doctrine of the Heraclitean flux was not to be found; 'he told the real truth' (not in the book, which is so entitled, but) 'privately to his disciples,' – words which imply that the connexion between the doctrines of Protagoras and Heracleitus was not generally recognized in Greece, but was really discovered or invented by Plato. On the other hand, the doctrine that 'Man is the measure of all things,' is expressly identified by Socrates with the other statement, that 'What appears to each man is to him;' and a reference is made to the books in which the statement occurs; – this Theaetetus, who has 'often read the books,' is supposed to acknowledge (so Cratylus). And Protagoras, in the speech attributed to him, never says that he has been misunderstood: he rather seems to imply that the absoluteness of sensation at each instant was to be found in his words. He is only indignant at the 'reductio ad absurdum' devised by Socrates for his 'homo mensura,' which Theodorus also considers to be 'really too bad.'
The question may be raised, how far Plato in the Theaetetus could have misrepresented Protagoras without violating the laws of dramatic probability. Could he have pretended to cite from a well-known writing what was not to be found there? But such a shadowy enquiry is not worth pursuing further. We need only remember that in the criticism which follows of the thesis of Protagoras, we are criticizing the Protagoras of Plato, and not attempting to draw a precise line between his real sentiments and those which Plato has attributed to him.
2. The other difficulty is a more subtle, and also a more important one, because bearing on the general character of the Platonic dialogues. On a first reading of them, we are apt to imagine that the truth is only spoken by Socrates, who is never guilty of a fallacy himself, and is the great detector of the errors and fallacies of others. But this natural presumption is disturbed by the discovery that the Sophists are sometimes in the right and Socrates in the wrong. Like the hero of a novel, he is not to be supposed always to represent the sentiments of the author. There are few modern readers who do not side with Protagoras, rather than with Socrates, in the dialogue which is called by his name. The Cratylus presents a similar difficulty: in his etymologies, as in the number of the State, we cannot tell how far Socrates is serious; for the Socratic irony will not allow him to distinguish between his real and his assumed wisdom. No one is the superior of the invincible Socrates in argument (except in the first part of the Parmenides, where he is introduced as a youth); but he is by no means supposed to be in possession of the whole truth. Arguments are often put into his mouth (compare Introduction to the Gorgias) which must have seemed quite as untenable to Plato as to a modern writer. In this dialogue a great part of the answer of Protagoras is just and sound; remarks are made by him on verbal criticism, and on the importance of understanding an opponent's meaning, which are conceived in the true spirit of philosophy. And the distinction which he is supposed to draw between Eristic and Dialectic, is really a criticism of Plato on himself and his own criticism of Protagoras.
The difficulty seems to arise from not attending to the dramatic character of the writings of Plato. There are two, or more, sides to questions; and these are parted among the different speakers. Sometimes one view or aspect of a question is made to predominate over the rest, as in the Gorgias or Sophist; but in other dialogues truth is divided, as in the Laches and Protagoras, and the interest of the piece consists in the contrast of opinions. The confusion caused by the irony of Socrates, who, if he is true to his character, cannot say anything of his own knowledge, is increased by the circumstance that in the Theaetetus and some other dialogues he is occasionally playing both parts himself, and even charging his own arguments with unfairness. In the Theaetetus he is designedly held back from arriving at a conclusion. For we cannot suppose that Plato conceived a definition of knowledge to be impossible. But this is his manner of approaching and surrounding a question. The lights which he throws on his subject are indirect, but they are not the less real for that. He has no intention of proving a thesis by a cut-and-dried argument; nor does he imagine that a great philosophical problem can be tied up within the limits of a definition. If he has analyzed a proposition or notion, even with the severity of an impossible logic, if half-truths have been compared by him with other half-truths, if he has cleared up or advanced popular ideas, or illustrated a new method, his aim has been sufficiently accomplished.
The writings of Plato belong to an age in which the power of analysis had outrun the means of knowledge; and through a spurious use of dialectic, the distinctions which had been already 'won from the void and formless infinite,' seemed to be rapidly returning to their original chaos. The two great speculative philosophies, which a century earlier had so deeply impressed the mind of Hellas, were now degenerating into Eristic. The contemporaries of Plato and Socrates were vainly trying to find new combinations of them, or to transfer them from the object to the subject. The Megarians, in their first attempts to attain a severer logic, were making knowledge impossible (compare Theaet.). They were asserting 'the one good under many names,' and, like the Cynics, seem to have denied predication, while the Cynics themselves were depriving virtue of all which made virtue desirable in the eyes of Socrates and Plato. And besides these, we find mention in the later writings of Plato, especially in the Theaetetus, Sophist, and Laws, of certain impenetrable godless persons, who will not believe what they 'cannot hold in their hands'; and cannot be approached in argument, because they cannot argue (Theat; Soph.). No school of Greek philosophers exactly answers to these persons, in whom Plato may perhaps have blended some features of the Atomists with the vulgar materialistic tendencies of mankind in general (compare Introduction to the Sophist).
And not only was there a conflict of opinions, but the stage which the mind had reached presented other difficulties hardly intelligible to us, who live in a different cycle of human thought. All times of mental progress are times of confusion; we only see, or rather seem to see things clearly, when they have been long fixed and defined. In the age of Plato, the limits of the world of imagination and of pure abstraction, of the old world and the new, were not yet fixed. The Greeks, in the fourth century before Christ, had no words for 'subject' and 'object,' and no distinct conception of them; yet they were always hovering about the question involved in them. The analysis of sense, and the analysis of thought, were equally difficult to them; and hopelessly confused by the attempt to solve them, not through an appeal to facts, but by the help of general theories respecting the nature of the universe.
Plato, in his Theaetetus, gathers up the sceptical tendencies of his age, and compares them. But he does not seek to reconstruct out of them a theory of knowledge. The time at which such a theory could be framed had not yet arrived. For there was no measure of experience with which the ideas swarming in men's minds could be compared; the meaning of the word 'science' could scarcely be explained to them, except from the mathematical sciences, which alone offered the type of universality and certainty. Philosophy was becoming more and more vacant and abstract, and not only the Platonic Ideas and the Eleatic Being, but all abstractions seemed to be at variance with sense and at war with one another.
The want of the Greek mind in the fourth century before Christ was not another theory of rest or motion, or Being or atoms, but rather a philosophy which could free the mind from the power of abstractions and alternatives, and show how far rest and how far motion, how far the universal principle of Being and the multitudinous principle of atoms, entered into the composition of the world; which could distinguish between the true and false analogy, and allow the negative as well as the positive a place in human thought. To such a philosophy Plato, in the Theaetetus, offers many contributions. He has followed philosophy into the region of mythology, and pointed out the similarities of opposing phases of thought. He has also shown that extreme abstractions are self-destructive, and, indeed, hardly distinguishable from one another. But his intention is not to unravel the whole subject of knowledge, if this had been possible; and several times in the course of the dialogue he rejects explanations of knowledge which have germs of truth in them; as, for example, 'the resolution of the compound into the simple;'