Theaetetus

by Plato

The relativity of knowledge is a truism to us, but was a great psychological discovery in the fifth century before Christ. Of this discovery, the first distinct assertion is contained in the thesis of Protagoras. Probably he had no intention either of denying or affirming an objective standard of truth. He did not consider whether man in the higher or man in the lower sense was a 'measure of all things.' Like other great thinkers, he was absorbed with one idea, and that idea was the absoluteness of perception. Like Socrates, he seemed to see that philosophy must be brought back from 'nature' to 'truth,' from the world to man. But he did not stop to analyze whether he meant 'man' in the concrete or man in the abstract, any man or some men, 'quod semper quod ubique' or individual private judgment. Such an analysis lay beyond his sphere of thought; the age before Socrates had not arrived at these distinctions. Like the Cynics, again, he discarded knowledge in any higher sense than perception. For 'truer' or 'wiser' he substituted the word 'better,' and is not unwilling to admit that both states and individuals are capable of practical improvement. But this improvement does not arise from intellectual enlightenment, nor yet from the exertion of the will, but from a change of circumstances and impressions; and he who can effect this change in himself or others may be deemed a philosopher. In the mode of effecting it, while agreeing with Socrates and the Cynics in the importance which he attaches to practical life, he is at variance with both of them. To suppose that practice can be divorced from speculation, or that we may do good without caring about truth, is by no means singular, either in philosophy or life. The singularity of this, as of some other (so-called) sophistical doctrines, is the frankness with which they are avowed, instead of being veiled, as in modern times, under ambiguous and convenient phrases.

Plato appears to treat Protagoras much as he himself is treated by Aristotle; that is to say, he does not attempt to understand him from his own point of view. But he entangles him in the meshes of a more advanced logic. To which Protagoras is supposed to reply by Megarian quibbles, which destroy logic, 'Not only man, but each man, and each man at each moment.' In the arguments about sight and memory there is a palpable unfairness which is worthy of the great 'brainless brothers,' Euthydemus and Dionysodorus, and may be compared with the egkekalummenos ('obvelatus') of Eubulides. For he who sees with one eye only cannot be truly said both to see and not to see; nor is memory, which is liable to forget, the immediate knowledge to which Protagoras applies the term. Theodorus justly charges Socrates with going beyond the truth; and Protagoras has equally right on his side when he protests against Socrates arguing from the common use of words, which 'the vulgar pervert in all manner of ways.'

III. The theory of Protagoras is connected by Aristotle as well as Plato with the flux of Heracleitus. But Aristotle is only following Plato, and Plato, as we have already seen, did not mean to imply that such a connexion was admitted by Protagoras himself. His metaphysical genius saw or seemed to see a common tendency in them, just as the modern historian of ancient philosophy might perceive a parallelism between two thinkers of which they were probably unconscious themselves. We must remember throughout that Plato is not speaking of Heracleitus, but of the Heracliteans, who succeeded him; nor of the great original ideas of the master, but of the Eristic into which they had degenerated a hundred years later. There is nothing in the fragments of Heracleitus which at all justifies Plato's account of him. His philosophy may be resolved into two elements—first, change, secondly, law or measure pervading the change: these he saw everywhere, and often expressed in strange mythological symbols. But he has no analysis of sensible perception such as Plato attributes to him; nor is there any reason to suppose that he pushed his philosophy into that absolute negation in which Heracliteanism was sunk in the age of Plato. He never said that 'change means every sort of change;' and he expressly distinguished between 'the general and particular understanding.' Like a poet, he surveyed the elements of mythology, nature, thought, which lay before him, and sometimes by the light of genius he saw or seemed to see a mysterious principle working behind them. But as has been the case with other great philosophers, and with Plato and Aristotle themselves, what was really permanent and original could not be understood by the next generation, while a perverted logic carried out his chance expressions with an illogical consistency. His simple and noble thoughts, like those of the great Eleatic, soon degenerated into a mere strife of words. And when thus reduced to mere words, they seem to have exercised a far wider influence in the cities of Ionia (where the people 'were mad about them') than in the life-time of Heracleitus—a phenomenon which, though at first sight singular, is not without a parallel in the history of philosophy and theology.

It is this perverted form of the Heraclitean philosophy which is supposed to effect the final overthrow of Protagorean sensationalism. For if all things are changing at every moment, in all sorts of ways, then there is nothing fixed or defined at all, and therefore no sensible perception, nor any true word by which that or anything else can be described. Of course Protagoras would not have admitted the justice of this argument any more than Heracleitus would have acknowledged the 'uneducated fanatics' who appealed to his writings. He might have said, 'The excellent Socrates has first confused me with Heracleitus, and Heracleitus with his Ephesian successors, and has then disproved the existence both of knowledge and sensation. But I am not responsible for what I never said, nor will I admit that my common-sense account of knowledge can be overthrown by unintelligible Heraclitean paradoxes.'

IV. Still at the bottom of the arguments there remains a truth, that knowledge is something more than sensible perception;—this alone would not distinguish man from a tadpole. The absoluteness of sensations at each moment destroys the very consciousness of sensations (compare Phileb.), or the power of comparing them. The senses are not mere holes in a 'Trojan horse,' but the organs of a presiding nature, in which they meet. A great advance has been made in psychology when the senses are recognized as organs of sense, and we are admitted to see or feel 'through them' and not 'by them,' a distinction of words which, as Socrates observes, is by no means pedantic. A still further step has been made when the most abstract notions, such as Being and Not-being, sameness and difference, unity and plurality, are acknowledged to be the creations of the mind herself, working upon the feelings or impressions of sense. In this manner Plato describes the process of acquiring them, in the words 'Knowledge consists not in the feelings or affections (pathemasi), but in the process of reasoning about them (sullogismo).' Here, is in the Parmenides, he means something not really different from generalization. As in the Sophist, he is laying the foundation of a rational psychology, which is to supersede the Platonic reminiscence of Ideas as well as the Eleatic Being and the individualism of Megarians and Cynics.

V. Having rejected the doctrine that 'Knowledge is perception,' we now proceed to look for a definition of knowledge in the sphere of opinion. But here we are met by a singular difficulty: How is false opinion possible? For we must either know or not know that which is presented to the mind or to sense. We of course should answer at once: 'No; the alternative is not necessary, for there may be degrees of knowledge; and we may know and have forgotten, or we may be learning, or we may have a general but not a particular knowledge, or we may know but not be able to explain;' and many other ways may be imagined in which we know and do not know at the same time. But these answers belong to a later stage of metaphysical discussion; whereas the difficulty in question naturally arises owing to the childhood of the human mind, like the parallel difficulty respecting Not-being. Men had only recently arrived at the notion of opinion; they could not at once define the true and pass beyond into the false. The very word doxa was full of ambiguity, being sometimes, as in the Eleatic philosophy, applied to the sensible world, and again used in the more ordinary sense of opinion. There is no connexion between sensible appearance and probability, and yet both of them met in the word doxa, and could hardly be disengaged from one another in the mind of the Greek living in the fifth or fourth century B.C. To this was often added, as at the end of the fifth book of the Republic, the idea of relation, which is equally distinct from either of them; also a fourth notion, the conclusion of the dialectical process, the making up of the mind after she has been 'talking to herself' (Theat.).

We are not then surprised that the sphere of opinion and of Not-being should be a dusky, half-lighted place (Republic), belonging neither to the old world of sense and imagination, nor to the new world of reflection and reason. Plato attempts to clear up this darkness. In his accustomed manner he passes from the lower to the higher, without omitting the intermediate stages. This appears to be the reason why he seeks for the definition of knowledge first in the sphere of opinion. Hereafter we shall find that something more than opinion is required.

False opinion is explained by Plato at first as a confusion of mind and sense, which arises when the impression on the mind does not correspond to the impression made on the senses. It is obvious that this explanation (supposing the distinction between impressions on the mind and impressions on the senses to be admitted) does not account for all forms of error; and Plato has excluded himself from the consideration of the greater number, by designedly omitting the intermediate processes of learning and forgetting; nor does he include fallacies in the use of language or erroneous inferences. But he is struck by one possibility of error, which is not covered by his theory, viz. errors in arithmetic. For in numbers and calculation there is no combination of thought and sense, and yet errors may often happen. Hence he is led to discard the explanation which might nevertheless have been supposed to hold good (for anything which he says to the contrary) as a rationale of error, in the case of facts derived from sense.

Another attempt is made to explain false opinion by assigning to error a sort of positive existence. But error or ignorance is essentially negative—a not-knowing; if we knew an error, we should be no longer in error. We may veil our difficulty under figures of speech, but these, although telling arguments with the multitude, can never be the real foundation of a system of psychology. Only they lead us to dwell upon mental phenomena which if expressed in an abstract form would not be realized by us at all. The figure of the mind receiving impressions is one of those images which have rooted themselves for ever in language. It may or may not be a 'gracious aid' to thought; but it cannot be got rid of. The other figure of the enclosure is also remarkable as affording the first hint of universal all-pervading ideas,—a notion further carried out in the Sophist. This is implied in the birds, some in flocks, some solitary, which fly about anywhere and everywhere. Plato discards both figures, as not really solving the question which to us appears so simple: 'How do we make mistakes?' The failure of the enquiry seems to show that we should return to knowledge, and begin with that; and we may afterwards proceed, with a better hope of success, to the examination of opinion.

But is true opinion really distinct from knowledge? The difference between these he seeks to establish by an argument, which to us appears singular and unsatisfactory. The existence of true opinion is proved by the rhetoric of the law courts, which cannot give knowledge, but may give true opinion. The rhetorician cannot put the judge or juror in possession of all the facts which prove an act of violence, but he may truly persuade them of the commission of such an act. Here the idea of true opinion seems to be a right conclusion from imperfect knowledge. But the correctness of such an opinion will be purely accidental; and is really the effect of one man, who has the means of knowing, persuading another who has not. Plato would have done better if he had said that true opinion was a contradiction in terms.