these utterly and
absolutely exclude the possibility of false opinion. The only cases, if
any, which remain, are the following.

THEAETETUS: What are they? If you tell me, I may perhaps understand you
better; but at present I am unable to follow you.

SOCRATES: A person may think that some things which he knows, or which
he perceives and does not know, are some other things which he knows and
perceives; or that some things which he knows and perceives, are other
things which he knows and perceives.

THEAETETUS: I understand you less than ever now.

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.

SOCRATES: The second case was, that I, knowing one of you and not
knowing the other, and perceiving neither, can never think him whom I
know to be him whom I do not know.

THEAETETUS: True.

SOCRATES: In the third case, not knowing and not perceiving either of
you, I cannot think that one of you whom I do not know is the other whom
I do not know. I need not again go over the catalogue of excluded cases,
in which I cannot form a false opinion about you and Theodorus, either
when I know both or when I am in ignorance of both, or when I know one
and not the other. And the same of perceiving: do you understand me?

THEAETETUS: I do.

SOCRATES: The only possibility of erroneous opinion is, when knowing you
and Theodorus, a