SOC. As the talk which the soul has with itself about any subjects which it considers. You must not suppose that I know this that I am declaring to you. But the soul, as the image presents itself to me, when it thinks, is merely conversing with itself, asking itself questions and answering,

 
affirming and denying. When it has arrived at a decision, whether slowly or with a sudden bound, and is at last agreed, and is not in doubt, we call that its opinion; and so I define forming opinion as talking and opinion as talk which has been held, not with someone else, nor yet aloud, but in silence with oneself. How do you define it? THEAET. In the same way. SOC. Then whenever a man has an opinion that one thing is another, he says to himself, we believe, that the one thing is the other. THEAET. Certainly. SOC. Now call to mind whether you have ever said to yourself that the beautiful is most assuredly ugly, or the wrong right, or—and this is the sum of the whole matter—consider whether you have ever tried to persuade yourself that one thing is most assuredly another, or whether quite the contrary is the case, and you have never ventured, even in sleep, to say to yourself that the odd is, after all, certainly even, or anything of that sort. THEAET. You are right. SOC. Do you imagine that anyone else, sane or insane, ever ventured to say to himself seriously and try to persuade himself that the ox must necessarily be a horse, or two one? THEAET. No, by Zeus, I do not. SOC. Then if forming opinion is talking to oneself, no one who talks and forms opinion of two objects and apprehends them both with his soul, could say and have the opinion that one is the other. But you will also have to give up the expression one and other. This is what I mean, that nobody holds the opinion that the ugly is beautiful, or
 
anything of that sort. THEAET. Well, Socrates, I do give it up; and I agree with you in what you say. SOC. You agree, therefore, that he who holds an opinion of both things cannot hold the opinion that one is the other. THEAET. So it seems. SOC. But surely he who holds an opinion of one only, and not of the other at all, will never hold the opinion that one is the other. THEAET. You are right; for he would be forced to apprehend also that of which he holds no opinion. SOC. Then neither he who holds opinion of both nor he who holds it of one can hold the opinion that a thing is something else. 
 
And so anyone who sets out to define false opinion as interchanged opinion would be talking nonsense. Then neither by this method nor by our previous methods is false opinion found to exist in us. THEAET. Apparently not. SOC. But yet, Theaetetus, if this is found not to exist, we shall be forced to admit many absurdities. THEAET. What absurdities?