The whole dialogue, as I said before, deserves to be studied because of its great influence on ancient and medieval thought; and this influence is not confined to what is least fantastic.
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CHAPTER XVIII Knowledge and Perception in Plato
M OST modern men take it for granted that empirical knowledge is dependent upon, or derived from, perception. There is, however, in Plato and among philosophers of certain other schools, a very different doctrine, to the effect that there is nothing worthy to be called «knowledge» to be derived from the senses, and that the only real knowledge has to do with concepts. In this view, «2 + 2 = 4» is genuine knowledge, but such a statement as «snow is white» is so full of ambiguity and uncertainty that it cannot find a place in the philosopher’s corpus of truths.
This view is perhaps traceable to Parmenides, but in its explicit form the philosophic world owes it to Plato. I propose, in this chapter, to deal only with Plato’s criticism of the view that knowledge is the same thing as perception, which occupies the first half of the Theaetetus.
This dialogue is concerned to find a definition of «knowledge,» but ends without arriving at any but a negative conclusion; several definitions are proposed and rejected, bur no definition that is considered satisfactory is suggested.
The first of the suggested definitions, and the only one that I shall consider, is set forth by Theaetetus in the words:
«It seems to me that one who knows something is perceiving the thing that he knows, and, so far as I can see at present, knowledge is nothing but perception.»
Socrates identifies this doctrine with that of Protagoras, that «man is the measure of all things,» i.e. that any given thing «is to me such as it appears to me, and is to you such as it appears to you.» Socrates adds: «Perception, then, is always something that is, and, as being knowledge, it is infallible.»
A large part of the argument that follows is concerned with the
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characterization of perception; when once this is completed, it does not take long to prove that such a thing as perception has turned out to be cannot be knowledge.
Socrates adds to the doctrine of Protagoras the doctrine of Heraclitus, that everything is always changing, i.e. that «all the things we are pleased to say ‘are’ really are in process of becoming.» Plato believes this to be true of objects of sense, but not of the objects of real knowledge. Throughout the dialogue, however, his positive doctrines remain in the background.
From the doctrine of Heraclitus, even if it be only applicable to objects of sense, together with the definition of knowledge as perception, it follows that knowledge is of what becomes, not of what is.
There are, at this point, some puzzles of a very elementary character. We are told that, since 6 is greater than 4 but less than 12, 6 is both great and small, which is a contradiction. Again, Socrates is now taller than Theaetetus, who is a youth not yet full grown; but in a few years Socrates will be shorter than Theaetetus. Therefore Socrates is both tall and short. The idea of a relational proposition seems to have puzzled Plato, as it did most of the great philosophers down to Hegel (inclusive). These puzzles, however, are not very germane to the argument, and may be ignored.
Returning to perception, it is regarded as due to an interaction between the object and the sense-organ, both of which, according to the doctrine of Heraclitus, are always changing, and both of which, in changing, change the percept. Socrates remarks that when he is well he finds wine sweet, but when ill, sour. Here it is a change in the percipient that causes the change in the percept.
Certain objections to the doctrine of Protagoras are advanced, and some of these are subsequently withdrawn. It is urged that Protagoras ought equally to have admitted pigs and baboons are measures of all things, since they also are percipients. Questions are raised as to the validity of perception in dreams and in madness. It is suggested that, if Protagoras is right, one man knows no more than another: not only is Protagoras as wise as the gods, but, what is more serious, he is no wiser than a fool. Further, if one man’s judgements are as correct as another’s, the people who judge that Protagoras is mistaken have the same reason to be thought right as he has.
Socrates undertakes to find an answer to many of these objections,
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putting himself, for the moment, in the place of Protagoras. As for dreams, the percepts are true as percepts. As for the argument about pigs and baboons, this is dismissed as vulgar abuse. As for the argument that, if each man is the measure of all things, one man is as wise as another, Socrates suggests, on behalf of Protagoras, a very interesting answer, namely that, while one judgement cannot be truer than another, it can be better, in the sense of having better
consequences. This suggests pragmatism. *
This answer, however, though Socrates has invented it, does not satisfy him. He urges, for example, that when a doctor foretells the course of my illness, he actually knows more of my future than I do. And when men differ as to what it is wise for the State to decree, the issue shows that some men had a greater knowledge as to the future than others had. Thus we cannot escape the conclusion that a wise man is a better measure of things than a fool.
All these are objections to the doctrine that each man is the measure of all things, and only indirectly to the doctrine that «knowledge» means «perception,» in so far as this doctrine leads to the other. There is, however, a direct argument, namely that memory must be allowed as well as perception. This is admitted, and to this extent the proposed definition is amended.
We come next to criticisms of the doctrine of Heraclitus. This is first pushed to extremes, in accordance, we are told, with the practice of his disciples among the bright youths of Ephesus. A thing may change in two ways, by locomotion, and by a change of quality, and the doctrine of flux is held to state that everything is always changing in both respects. †And not only is everything always undergoing some qualitative change, but everything is always changing all its qualities—so, we are told, clever people think at Ephesus. This has
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* It was presumably this passage that first suggested to F. C. S. Schiller his admiration of Protagoras.
â It seems that neither Plato nor the dynamic youths of Ephesus had noticed that locomotion is impossible on the extreme Heraclitean doctrine. Motion demands that a given thing A should be now here, now there; it must remain the same thing while it moves. In the doctrine that Plato examines there is change of quality and change of place, but not change of substance. In this respect, modern quantum physics goes further than the most extreme disciples of Heraclitus went in Plato’s time. Plato would have thought this fatal to science, but it has not proved so.
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awkward consequences. We cannot say «this is white,» for if it was white when we began speaking it will have ceased to be white before we end our sentence. We cannot be right in saying we are seeing a thing, for seeing is perpetually changing into not-seeing. * If everything is changing in every kind of way, seeing has no right to be called seeing rather than not-seeing, or perception to be called perception rather than not-perception. And when we say «perception is knowledge,» we might just as well say «perception is not-knowledge.»
What the above argument amounts to is that, whatever else may be in perpetual flux, the meanings of words must be fixed, at least for a time, since otherwise no assertion is definite, and no assertion is true rather than false. There must be something more or less constant, if discourse and knowledge are to be possible. This, I think, should be admitted. But a great deal of flux is compatible with this admission.
There is, at this point, a refusal to discuss Parmenides, on the ground that he is too great and grand. He is a «reverend and awful figure.» «There was a sort of depth in him that was altogether noble.» He is «one being whom I respect above all.» In these remarks Plato shows his love for a static universe, and his dislike of the Heraclitean flux which he has been admitting for the sake of argument. But after this expression of reverence he abstains from developing the Parmenidean alternative to Heraclitus.
We now reach Plato’s final argument against the identification of knowledge with perception. He begins by pointing out that we perceive through eyes and ears, rather than with them, and he goes on to point out that some of our knowledge is not connected with any sense-organ. We can know, for instance, that sounds and colours
