SOCRATES: Hear me once more, then:—I, knowing Theodorus, and remembering in my own mind what sort of person he is, and also what sort of person Theaetetus is, at one time see them, and at another time do not see them, and sometimes I touch them, and at another time not, or at one time I may hear them or perceive them in some other way, and at another time not perceive them, but still I remember them, and know them in my own mind.
THEAETETUS: Very true.
SOCRATES: Then, first of all, I want you to understand that a man may or may not perceive sensibly that which he knows.
THEAETETUS: True.
SOCRATES: And that which he does not know will sometimes not be perceived by him and sometimes will be perceived and only perceived?
THEAETETUS: That is also true.
SOCRATES: See whether you can follow me better now: Socrates can recognize Theodorus and Theaetetus, but he sees neither of them, nor does he perceive them in any other way; he cannot then by any possibility imagine in his own mind that Theaetetus is Theodorus. Am I not right?
THEAETETUS: You are quite right.
SOCRATES: Then that was the first case of which I spoke.
THEAETETUS: Yes.