SOC. And, indeed, if I may venture to say so, it is not a bad description of knowledge

 
that you have given, but one which Protagoras also used to give. Only, he has said the same thing in a different way. For he says somewhere that man is the measure of all things, of the existence of the things that are and the non-existence of the things that are not. You have read that, I suppose? THEAET. Yes, I have read it often. SOC. Well, is not this about what he means, that individual things are for me such as they appear to me, and for you in turn such as they appear to you —you and I being man ? THEAET. Yes, that is what he says. SOC. It is likely that a wise man is not talking nonsense; so let us follow after him. Is it not true that sometimes, when the same wind blows, one of us feels cold, and the other does not? or one feels slightly and the other exceedingly cold? THEAET. Certainly. SOC. Then in that case, shall we say that the wind is in itself cold or not cold or shall we accept Protagoras’s saying that it is cold for him who feels cold and not for him who does not? THEAET. Apparently we shall accept that. SOC. Then it also seems cold, or not, to each of the two? THEAET. Yes. SOC. But seems denotes perceiving? THEAET. It does. SOC. Then seeming and perception are the same thing in matters of warmth and everything of that sort. For as each person perceives things, such they are to each person. THEAET. Apparently. SOC. Perception, then, is always of that which exists and, since it is knowledge, cannot be false. THEAET. So it seems. SOC. By the Graces! I wonder if Protagoras, who was a very wise man, did not utter this dark saying to the common herd like ourselves, and tell the truth An allusion to the title of Protagoras’s book, Truth . in secret to his pupils. THEAET. Why, Socrates, what do you mean by that? SOC. I will tell you and it is not a bad description, either, that nothing is one and invariable, and you could not rightly ascribe any quality whatsoever to anything, but if you call it large it will also appear to be small, and light if you call it heavy, and everything else in the same way, since nothing whatever is one, either a particular thing or of a particular quality; but it is out of movement and motion and mixture with one another that all those things become which we wrongly say are —wrongly, because nothing ever is, but is always becoming. 
 
And on this subject all the philosophers, except Parmenides, may be marshalled in one line—Protagoras and Heracleitus and Empedocles—and the chief poets in the two kinds of poetry, Epicharmus, in comedy, and in tragedy, Homer, who, in the line 0ceanus the origin of the gods, and Tethys their mother, Hom. Il. 14.201, 302 has said that all things are the offspring of flow and motion; or don’t you think he means that? THEAET. I think he does. SOC. Then who could still contend with such a great host,

 
led by Homer as general, and not make himself ridiculous? THEAET. It is not easy, Socrates. SOC. No, Theaetetus, it is not. For the doctrine is amply proved by this, namely, that motion is the cause of that which passes for existence, that is, of becoming, whereas rest is the cause of non-existence and destruction; for warmth or fire, which, you know, is the parent and preserver of all other things, is itself the offspring of movement and friction, and these two are forms of motion. Or are not these the source of fire? THEAET. Yes, they are. SOC. And furthermore, the animal kingdom is sprung from these same sources. THEAET. Of course. SOC. Well, then, is not the bodily habit destroyed by rest and idleness, and preserved, generally speaking, by gymnastic exercises and motions? THEAET. Yes. SOC. And what of the habit of the soul? Does not the soul acquire information and is it not preserved and made better through learning and practice, which are motions, whereas through rest, which is want of practice and of study,
 
it learns nothing and forgets what it has learned? THEAET. Certainly. SOC. Then the good, both for the soul and for the body, is motion, and rest is the opposite? THEAET. Apparently. SOC. Now shall I go on and mention to you also windless air, calm sea, and all that sort of thing, and say that stillness causes decay and destruction and that the opposite brings preservation? And shall I add to this the all-compelling and crowning argument that Homer by the golden chain Hom. Il. 8.18 ff. especially 26. In this passage Zeus declares that all the gods and goddesses together could not, with a golden chain, drag him from on high, but that if he pulled, he would drag them, with earth and sea, would then bind the chain round the summit of Olympus, and all the rest would hang aloft. This crowning argument is a reductio ad absurdum of the habit of using texts from Homer in support of all kinds of doctrine. refers to nothing else than the sun,
 
and means that so long as the heavens and the sun go round everything exists and is preserved, among both gods and men, but if the motion should stop, as if bound fast, everything would be destroyed and would, as the saying is, be turned upside down? THEAET. Yes, Socrates, I think he means what you say he does. SOC. Then, my friend, you must apply the doctrine in this way: first as concerns vision, the color that you call white is not to be taken as something separate outside of your eyes, nor yet as something inside of them; and you must not assign any place to it,
 
for then it would at once be in a definite position and stationary and would have no part in the process of becoming. THEAET. But what do you mean? SOC. Let us stick close to the statement we made a moment ago, and assume that nothing exists by itself as invariably one: then it will be apparent that black or white or any other color whatsoever is the result of the impact of the eye upon the appropriate motion, and therefore that which we call color

 
will be in each instance neither that which impinges nor that which is impinged upon, but something between, which has occurred, peculiar to each individual. Or would you maintain that each color appears to a dog, or any other animal you please, just as it does to you? THEAET. No, by Zeus, I wouldn’t. SOC. Well, does anything whatsoever appear the same to any other man as to you? Are you sure of this? Or are you not much more convinced that nothing appears the same even to you, because you yourself are never exactly the same? THEAET. Yes, I am much more convinced of the last. SOC. Then, if that with which I compare myself in size, or which I touch,
 
were really large or white or hot, it would never have become different by coming in contact with something different, without itself changing; and if, on the other hand, that which did the comparing or the touching were really large or white or hot, it would not have become different when something different approached it or was affected in some way by it, without being affected in some way itself. For nowadays, my friend, we find ourselves rather easily forced to make extraordinary and absurd statements, as Protagoras and everyone who undertakes to agree with him would say. THEAET. What do you mean? What statements? SOC. Take a little example and you will know all I have in mind. Given six dice, for instance, if you compare four with them, we say that they are more than the four, half as many again, but if you compare twelve with them, we say they are less, half as many; and any other statement would be inadmissible; or would you admit any other? THEAET. Not I. SOC. Well then, if Protagoras, or anyone else, ask you, Theaetetus, can anything become greater or more in any other way than by being increased? what reply will you make? THEAET. If I am to say what I think, Socrates, with reference to
 
the present question, I should say no, but if I consider the earlier question, I should say yes, for fear of contradicting myself. SOC. Good, by Hera! Excellent, my friend! But apparently, if you answer yes it will be in the Euripidean spirit; for our tongue will be unconvinced, but not our mind. Eur. Hipp. 612 ἡ γλῶσσ’ ὀμώμοχ’, ἡ δὲ φρὴν ἀνώμοτος , my tongue has sworn, but my mind is unsworn. THEAET. True. SOC. Well, if you and I were clever and wise and had found out everything about the mind, we should henceforth spend the rest of our time testing each other out of the fulness of our wisdom,
 
rushing together like sophists in a sophistical combat, battering each other’s arguments with counter arguments. But, as it is, since we are ordinary people, we shall wish in the first place to look into the real essence of our thoughts and see whether they harmonize with one another or not at all. THEAET. Certainly that is what I should like. SOC. And so should I. But since this is the case, and we have plenty of time, shall we not quietly, without any impatience, but truly examining ourselves,

 
consider again the nature of these appearances within us? And as we consider them, I shall say, I think, first, that nothing can ever become more or less in size or number, so long as it remains equal to itself. Is it not so? THEAET. Yes. SOC. And secondly, that anything to which nothing is added and from which nothing is subtracted, is neither increased nor diminished, but is always equal. THEAET. Certainly. SOC. And should we not say thirdly, that what was not previously could not afterwards be without becoming and having become? THEAET. Yes, I agree. SOC. These three assumptions contend with one another in our minds when we talk about the dice, or when we say that I, who do not, at my age, either increase in size or diminish, am in the course of a year first larger than you, who are young, and afterwards smaller, when nothing has been taken from my size,
 
but you have grown. For I am, it seems, afterwards what I was not before, and I have not become so; for it is impossible to have become without becoming, and without losing anything of my size I could not become smaller. And there are countless myriads of such contradictions, if we are to accept these that I have mentioned. You follow me, I take it, Theaetetus, for I think you are not new at such things. THEAET. By the gods, Socrates, I am lost in wonder when I think of all these things, and sometimes when I regard them it really makes my head swim. SOC. Theodorus seems to be a pretty good guesser about your nature. For this feeling of wonder shows that you are a philosopher, since wonder is the only beginning of philosophy, and he who said that Iris was the child of Thaumas Hes. Theog. 750 Iris is the messenger of heaven, and Plato interprets the name of her father as Wonder ( θαῦμα ). made a good genealogy. But do you begin to understand why these things are so, according to the doctrine we attribute to Protagoras, or do you not as yet? THEAET. Not yet, I think. SOC. And will you be grateful to me if I help you
 
to search out the hidden truth of the thought of a famous man or, I should say, of famous men? THEAET. Of course I shall be grateful, very grateful. SOC. Look round and see that none of the uninitiated is listening. The uninitiated are those who think nothing is except what they can grasp firmly with their hands, and who deny the existence of actions and generation and all that is invisible. THEAET. Truly, Socrates, those you speak of are very stubborn

 
and perverse mortals. SOC. So they are, my boy, quite without culture. But others are more clever, whose secret doctrines I am going to disclose to you. For them the beginning, upon which all the things we were just now speaking of depend, is the assumption that everything is real motion and that there is nothing besides this, but that there are two kinds of motion, each infinite in the number of its manifestations, and of these kinds one has an active, the other a passive force. From the union and friction of these two are born offspring, infinite in number, but always twins, the object of sense
 
and the sense which is always born and brought forth together with the object of sense. Now we give the senses names like these: sight and hearing and smell, and the sense of cold and of heat, and pleasures and pains and desires and fears and so forth. Those that have names are very numerous, and those that are unnamed are innumerable. Now the class of objects of sense is akin to each of these; all sorts of colors are akin to all sorts of acts of vision, and in the same way sounds to acts of hearing,
 
and the other objects of sense spring forth akin to the other senses. What does this tale mean for us, Theaetetus, with reference to what was said before? Do you see? THEAET. Not quite, Socrates. SOC. Just listen; perhaps we can finish the tale. It means, of course, that all these things are, as we were saying, in motion, and their motion has in it either swiftness or slowness. Now the slow element keeps its motion in the same place and directed towards such things as draw near it, and indeed it is in this way that it begets. 
 
But the things begotten in this way are quicker; for they move from one place to another, and their motion is naturally from one place to another. Now when the eye and some appropriate object which approaches beget whiteness and the corresponding perception—which could never have been produced by either of them going to anything else—then, while sight from the eye and
 
whiteness from that which helps to produce the color are moving from one to the other, the eye becomes full of sight and so begins at that moment to see, and becomes, certainly not sight, but a seeing eye, and the object which joined in begetting the color is filled with whiteness and becomes in its turn, not whiteness, but white, whether it be a stick or a stone, or whatever it be the hue of which is so colored.