THEAETETUS: That, Socrates, is impossible.
SOCRATES: But perhaps he thinks of something which he does not know as some other thing which he does not know; for example, he knows neither Theaetetus nor Socrates, and yet he fancies that Theaetetus is Socrates, or Socrates Theaetetus?
THEAETETUS: How can he?
SOCRATES: But surely he cannot suppose what he knows to be what he does not know, or what he does not know to be what he knows?
THEAETETUS: That would be monstrous.
SOCRATES: Where, then, is false opinion? For if all things are either known or unknown, there can be no opinion which is not comprehended under this alternative, and so false opinion is excluded.
THEAETETUS: Most true.
SOCRATES: Suppose that we remove the question out of the sphere of knowing or not knowing, into that of being and not-being.
THEAETETUS: What do you mean?
SOCRATES: May we not suspect the simple truth to be that he who thinks about anything, that which is not, will necessarily think what is false, whatever in other respects may be the state of his mind?
THEAETETUS: That, again, is not unlikely, Socrates.
SOCRATES: Then suppose some one to say to us, Theaetetus:--Is it possible for any man to think that which is not, either as a self-existent substance or as a predicate of something else? And suppose that we answer, "Yes, he can, when he thinks what is not true."--That will be our answer?
THEAETETUS: Yes.
SOCRATES: But is there any parallel to this?
THEAETETUS: What do you mean?
SOCRATES: Can a man see something and yet see nothing?
THEAETETUS: Impossible.
SOCRATES: But if he sees any one thing, he sees something that exists.
Do you suppose that what is one is ever to be found among non-existing things?
THEAETETUS: I do not.
SOCRATES: He then who sees some one thing, sees something which is?
THEAETETUS: Clearly.
SOCRATES: And he who hears anything, hears some one thing, and hears that which is?
THEAETETUS: Yes.
SOCRATES: And he who touches anything, touches something which is one and therefore is?
THEAETETUS: That again is true.
SOCRATES: And does not he who thinks, think some one thing?
THEAETETUS: Certainly.
SOCRATES: And does not he who thinks some one thing, think something which is?
THEAETETUS: I agree.
SOCRATES: Then he who thinks of that which is not, thinks of nothing?
THEAETETUS: Clearly.
SOCRATES: And he who thinks of nothing, does not think at all?
THEAETETUS: Obviously.
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 n.o.ble, or the n.o.ble 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?