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.
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 (Both words in Greek are called eteron: compare Parmen.; Euthyd.)). 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.