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Very true, he said.

Then the wise or temperate man, and he only, will know himself; and be able to examine what he knows or docs not know, and to see what others know and think that they know and do really know; an<l 11at they do not know, and fancy that thLy know, when they do not. No other person will be able to do this. And this is wisdom and temperance and self-knowledge for a man to know what he knows, and what he does not know. That is your meaning?

Ycs, he said.

Now then, f said, making an offering of the third or last argument to Zeus the Saviour, let us bLgin again, and ask, in the first place, whether it is or is not possible for a person to know that he knows and docs not knr, if perfi ctly possible, such knowledge is of any use.

That is what we have to com,idu, he said.

An<l here, Critias, I said, I hope that you will find a way out of a difficulty into which I have got myself. Shall I tell you the natm e of the difficulty?

By all means, he replied.

Does not what you have been saying, if true, amount to this: that there must be a single science which is wholly a science of itself and of other sciences, and that the same is also the science of the absence of science?

Yes.

But consider how monstrous this proposition is, my friend: in any parallel case, the impossibility will be transparent to you.

How is that? and in what cases do you mean?

In such cases as this: Suppose that there is a kind of vision which is not like ordinary vision, but a vision of itself and of other sorts of vision, and of the defect of them, which in seeing sees no colour, but only itself and other sorts of vision: Do you think that there is such a kind of vision?

Certainly not.

Or is there a kind of hearing which hears no sound at all, but only itself and other sorts of hearing, or the defects of them?

There is not.