Socrates: You think, if I am not mistaken, that your “truly false” is safe from censure, and that I shall never ask whether there can be a swift which is slow, or a heavy which is light, or any other self-contradictory thing, which works, not according to its own nature, but according to that of its opposite. But I will not insist upon this, for I do not wish needlessly to discourage you. And so you are satisfied that false opinion is heterodoxy, or the thought of something else?
Theaetetus: I am.
Socrates: It is possible then upon your view for the mind to conceive of one thing as another?
Theaetetus: True.
Socrates: But must not the mind, or thinking power, which misplaces them, have a conception either of both objects or of one of them?
Theaetetus: Certainly.
Socrates: Either together or in succession?
Theaetetus: Very good.
Socrates: And do you mean by conceiving, the same which I mean?
Theaetetus: What is that?
Socrates: I mean the conversation which the soul holds with herself in considering of anything. I speak of what I scarcely understand; but the soul when thinking appears to me to be just talking—asking questions of herself and answering them, affirming and denying. And when she has arrived at a decision, either gradually or by a sudden impulse, and has at last agreed, and does not doubt, this is called her opinion. I say, then, that to form an opinion is to speak, and opinion is a word spoken—I mean, to oneself and in silence, not aloud or to another: What think you?
Theaetetus: I agree.
Socrates: Then when anyone thinks of one thing as another, he is saying to himself that one thing is another?
Theaetetus: Yes.
Socrates: But do you ever remember saying to yourself that the noble is certainly base, or the unjust just; or, best of all—have you ever attempted to convince yourself that one thing is another? Nay, not even in sleep, did you ever venture to say to yourself that odd is even, or anything of the kind?
Theaetetus: Never.
Socrates: And do you suppose that any other man, either in his senses or out of them, ever seriously tried to persuade himself that an ox is a horse, or that two are one?
Theaetetus: Certainly not.
Socrates: But if thinking is talking to oneself, no one speaking and thinking of two objects, and apprehending them both in his soul, will say and think that the one is the other of them, and I must add, that even you, lover of dispute as you are, had better let the word “other” alone (i.e. not insist that “one” and “other” are the same). I mean to say, that no one thinks the noble to be base, or anything of the kind.
Theaetetus: I will give up the word “other,” Socrates; and I agree to what you say.
Socrates: If a man has both of them in his thoughts, he cannot think that the one of them is the other?
Theaetetus: True.
Socrates: Neither, if he has one of them only in his mind and not the other, can he think that one is the other?
Theaetetus: True; for we should have to suppose that he apprehends that which is not in his thoughts at all.
Socrates: Then no one who has either both or only one of the two objects in his mind can think that the one is the other. And therefore, he who maintains that false opinion is heterodoxy is talking nonsense; for neither in this, any more than in the previous way, can false opinion exist in us.