Theaetetus (158)

THEAET. What you say is very true, Socrates.

SOC. What argument is left, then, my boy, for the man who says that perception is knowledge and that in each case the things which appear are to the one to whom they appear?

THEAET. I hesitate to say, Socrates, that I have no reply to make, because you scolded me just now when I said that. But really I cannot dispute that those who are insane or dreaming have false opinions, when some of them think they are gods and others fancy in their sleep that they have wings and are flying.

SOC. Don’t you remember, either, the similar dispute about these errors, especially about sleeping and waking?

THEAET. What dispute?

SOC. One which I fancy you have often heard. The question is asked, what proof you could give if anyone should ask us now, at the present moment, whether we are asleep and our thoughts are a dream, or whether we are awake and talking with each other in a waking condition.

THEAET. Really, Socrates, I don’t see what proof can be given; for there is an exact correspondence in all particulars, as between the strophe and antistrophe of a choral song. Take, for instance, the conversation we have just had: there is nothing to prevent us from imagining in our sleep also that we are carrying on this conversation with each other, and when in a dream we imagine that we are relating dreams, the likeness between the one talk and the other is remarkable.

SOC. So you see it is not hard to dispute the point, since it is even open to dispute whether we are awake or in a dream. Now since the time during which we are asleep is equal to that during which we are awake, in each state our spirit contends that the semblances that appear to it at any time are certainly true, so that for half the time we say that this is true, and for half the time the other, and we maintain each with equal confidence.

THEAET. Certainly.

SOC. And may not, then, the same be said about insanity and the other diseases, except that the time is not equal?

THEAET. Yes.

SOC. Well, then, shall truth be determined by the length or shortness of time?

THEAET. That would be absurd in many ways.

SOC. But can you show clearly in any other way which of the two sets of opinions is true?

THEAET. I do not think I can.

SOC. Listen, then, while I tell you what would be said about them by those who maintain that what appears at any time is true for him to whom it appears. They begin, I imagine, by asking this question: Theaetetus, can that which is wholly other have in any way the same quality as its alternative? And we must not assume that the thing in question is partially the same and partially other, but wholly other.