SOCRATES: Then no one can think that which is not, either as a self-existent substance or as a predicate of something else? THEAETETUS: Clearly not. SOCRATES: Then to think falsely is different from thinking that which is not? THEAETETUS: It would seem so. SOCRATES: Then false opinion has no existence in us, either in the sphere of being or of knowledge? THEAETETUS: Certainly not. SOCRATES: But may not the following be the description of what we express by this name? THEAETETUS: What? SOCRATES: May we not suppose that false opinion or thought is a sort of heterodoxy; a person may make an exchange in his mind, and say that one real object is another real object. For thus he always thinks that which is, but he puts one thing in place of another; and missing the aim of his thoughts, he may be truly said to have false opinion. THEAETETUS: Now you appear to me to have spoken the exact truth: when a man puts the base in the place of the noble, or the noble in the place of the base, then he has truly false opinion. SOCRATES: I see, Theaetetus, that your fear has disappeared, and that you are beginning to despise me. THEAETETUS: What makes you say so? 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 any one thinks of one thing as another, he is saying to himself that one thing is another? THEAETETUS: Yes.