Theaetetus (193)

SOC. See if you follow me better now. If Socrates knows Theodorus and Theaetetus, but sees neither of them and has no other perception of them, he never could have the opinion within himself that Theaetetus is Theodorus. Am I right or wrong?

THEAET. You are right.

SOC. Now that was the first of the cases of which I spoke.

THEAET. Yes, it was.

SOC. The second is this: knowing one of you and not knowing the other, and not perceiving either of you, I never could think that the one whom I know is the one whom I do not know.

THEAET. Right.

SOC. And this is the third case: not knowing and not perceiving either of you, I could not think that he whom I do not know is someone else whom I do not know. And imagine that you have heard all the other cases again in succession, in which I could never form false opinions about you and Theodorus, either when I know or do not know both of you, or when I know one and not the other; and the same is true if we say perceive instead of know. Do you follow me?

THEAET. I follow you.

SOC. Then the possibility of forming false opinion remains in the following case: when, for example, knowing you and Theodorus, and having on that block of wax the imprint of both of you, as if you were signet-rings, but seeing you both at a distance and indistinctly, I hasten to assign the proper imprint of each of you to the proper vision, and to make it fit, as it were, its own footprint, with the purpose of causing recognition; [*] but I may fail in this by interchanging them, and put the vision of one upon the imprint of the other, as people put a shoe on the wrong foot; or, again, I may be affected as the sight is affected when we use a mirror and the sight as it flows makes a change from right to left, and thus make a mistake; it is in such cases, then, that interchanged opinion occurs and the forming of false opinion arises.

THEAET. I think it does, Socrates. You describe what happens to opinion marvelously well.

SOC. There is still the further case, when, knowing both of you, I perceive one in addition to knowing him, but do not perceive the other, and the knowledge which I have of that other is not in accord with my perception. This is the case I described in this way before, and at that time you did not understand me.

THEAET. No, I did not.

SOC. This is what I meant, that if anyone knows and perceives one of you, and has knowledge of him which accords with the perception, he will never think that he is someone else whom he knows and perceives and his knowledge of whom accords with the perception. That was the case, was it not?

THEAET. Yes.