And did we not say, that all sensations arise thus: they move about between the agent and patient together with a perception, and the patient ceases to be a perceiving power and becomes a percipient, and the agent a quale instead of a quality; but neither has any absolute existence? But now we make the further discovery, that neither white or whiteness, nor any sense or sensation, can be predicated of anything, for they are in a perpetual flux. And therefore we must modify the doctrine of Theaetetus and Protagoras, by asserting further that knowledge is and is not sensation; and of everything we must say equally, that this is and is not, or becomes or becomes not. And still the word 'this' is not quite correct, for language fails in the attempt to express their meaning.
At the close of the discussion, Theodorus claims to be released from the argument, according to his agreement. But Theaetetus insists that they shall proceed to consider the doctrine of rest. This is declined by Socrates, who has too much reverence for the great Parmenides lightly to attack him. (We shall find that he returns to the doctrine of rest in the Sophist; but at present he does not wish to be diverted from his main purpose, which is, to deliver Theaetetus of his conception of knowledge.) He proceeds to interrogate him further. When he says that 'knowledge is in perception,' with what does he perceive? The first answer is, that he perceives sights with the eye, and sounds with the ear. This leads Socrates to make the reflection that nice distinctions of words are sometimes pedantic, but sometimes necessary; and he proposes in this case to substitute the word 'through' for 'with.' For the senses are not like the Trojan warriors in the horse, but have a common centre of perception, in which they all meet. This common principle is able to compare them with one another, and must therefore be distinct from them (compare Republic). And as there are facts of sense which are perceived through the organs of the body, there are also mathematical and other abstractions, such as sameness and difference, likeness and unlikeness, which the soul perceives by herself. Being is the most universal of these abstractions. The good and the beautiful are abstractions of another kind, which exist in relation and which above all others the mind perceives in herself, comparing within her past, present, and future. For example; we know a thing to be hard or soft by the touch, of which the perception is given at birth to men and animals. But the essence of hardness or softness, or the fact that this hardness is, and is the opposite of softness, is slowly learned by reflection and experience. Mere perception does not reach being, and therefore fails of truth; and therefore has no share in knowledge. But if so, knowledge is not perception. What then is knowledge? The mind, when occupied by herself with being, is said to have opinion – shall we say that 'Knowledge is true opinion'? But still an old difficulty recurs; we ask ourselves, 'How is false opinion possible?' This difficulty may be stated as follows: —
Either we know or do not know a thing (for the intermediate processes of learning and forgetting need not at present be considered); and in thinking or having an opinion, we must either know or not know that which we think, and we cannot know and be ignorant at the same time; we cannot confuse one thing which we do not know, with another thing which we do not know; nor can we think that which we do not know to be that which we know, or that which we know to be that which we do not know. And what other case is conceivable, upon the supposition that we either know or do not know all things? Let us try another answer in the sphere of being: 'When a man thinks, and thinks that which is not.' But would this hold in any parallel case? Can a man see and see nothing? or hear and hear nothing? or touch and touch nothing? Must he not see, hear, or touch some one existing thing? For if he thinks about nothing he does not think, and not thinking he cannot think falsely. And so the path of being is closed against us, as well as the path of knowledge. But may there not be 'heterodoxy,' or transference of opinion; – I mean, may not one thing be supposed to be another? Theaetetus is confident that this must be 'the true falsehood,' when a man puts good for evil or evil for good. Socrates will not discourage him by attacking the paradoxical expression 'true falsehood,' but passes on. The new notion involves a process of thinking about two things, either together or alternately. And thinking is the conversing of the mind with herself, which is carried on in question and answer, until she no longer doubts, but determines and forms an opinion. And false opinion consists in saying to yourself, that one thing is another. But did you ever say to yourself, that good is evil, or evil good? Even in sleep, did you ever imagine that odd was even? Or did any man in his senses ever fancy that an ox was a horse, or that two are one? So that we can never think one thing to be another; for you must not meet me with the verbal quibble that one – eteron – is other – eteron (both 'one' and 'other' in Greek are called 'other' – eteron). He who has both the two things in his mind, cannot misplace them; and he who has only one of them in his mind, cannot misplace them – on either supposition transplacement is inconceivable.
But perhaps there may still be a sense in which we can think that which we do not know to be that which we know: e.g. Theaetetus may know Socrates, but at a distance he may mistake another person for him. This process may be conceived by the help of an image. Let us suppose that every man has in his mind a block of wax of various qualities, the gift of Memory, the mother of the Muses; and on this he receives the seal or stamp of those sensations and perceptions which he wishes to remember. That which he succeeds in stamping is remembered and known by him as long as the impression lasts; but that, of which the impression is rubbed out or imperfectly made, is forgotten, and not known. No one can think one thing to be another, when he has the memorial or seal of both of these in his soul, and a sensible impression of neither; or when he knows one and does not know the other, and has no memorial or seal of the other; or when he knows neither; or when he perceives both, or one and not the other, or neither; or when he perceives and knows both, and identifies what he perceives with what he knows (this is still more impossible); or when he does not know one, and does not know and does not perceive the other; or does not perceive one, and does not know and does not perceive the other; or has no perception or knowledge of either – all these cases must be excluded. But he may err when he confuses what he knows or perceives, or what he perceives and does not know, with what he knows, or what he knows and perceives with what he knows and perceives.
Theaetetus is unable to follow these distinctions; which Socrates proceeds to illustrate by examples, first of all remarking, that knowledge may exist without perception, and perception without knowledge. I may know Theodorus and Theaetetus and not see them; I may see them, and not know them. 'That I understand.' But I could not mistake one for the other if I knew you both, and had no perception of either; or if I knew one only, and perceived neither; or if I knew and perceived neither, or in any other of the excluded cases. The only possibility of error is: 1st, when knowing you and Theodorus, and having the impression of both of you on the waxen block, I, seeing you both imperfectly and at a distance, put the foot in the wrong shoe – that is to say, put the seal or stamp on the wrong object: or 2ndly, when knowing both of you I only see one; or when, seeing and knowing you both, I fail to identify the impression and the object. But there could be no error when perception and knowledge correspond.
The waxen block in the heart of a man's soul, as I may say in the words of Homer, who played upon the words ker and keros, may be smooth and deep, and large enough, and then the signs are clearly marked and lasting, and do not get confused. But in the 'hairy heart,' as the all-wise poet sings, when the wax is muddy or hard or moist, there is a corresponding confusion and want of retentiveness; in the muddy and impure there is indistinctness, and still more in the hard, for there the impressions have no depth of wax, and in the moist they are too soon effaced. Yet greater is the indistinctness when they are all jolted together in a little soul, which is narrow and has no room. These are the sort of natures which have false opinion; from stupidity they see and hear and think amiss; and this is falsehood and ignorance. Error, then, is a confusion of thought and sense.
Theaetetus is delighted with this explanation. But Socrates has no sooner found the new solution than he sinks into a fit of despondency. For an objection occurs to him: – May there not be errors where there is no confusion of mind and sense? e.g. in numbers. No one can confuse the man whom he has in his thoughts with the horse which he has in his thoughts, but he may err in the addition of five and seven. And observe that these are purely mental conceptions. Thus we are involved once more in the dilemma of saying, either that there is no such thing as false opinion, or that a man knows what he does not know.
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