nd temperance and
self-knowledge--for a man to know what he knows, and what he does not
know. That is your meaning?

Yes, he said.

Now then, I said, making an offering of the third or last argument
to Zeus the Saviour, let us begin again, and ask, in the first place,
whether it is or is not possible for a person to know that he knows and
does not know what he knows and does not know; and in the second place,
whether, if perfectly possible, such knowledge is of any use.

That is what we have to consider, he said.

And 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 nature 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.

Or take all the senses: can you imagine that there is any sense of
itself and of other senses, but which is incapable of perceiving the
objects of the senses?

I think not.

Could there be any desire which is not the desire of any pleasure, but
of itself, and of all other desires?

Certainly not.

Or can you imagine a wish which wishes for no good, but only for itself
and all other wishes?

I should answer, No.

Or would you say that there is a love which is not the love of beauty,
but of itself and of other loves?

I should not.

Or did you ever know of a fear which fears itself or other fears, but
has no object of fear?

I never did, he said.

Or of an opinion which is an opinion of itself and of other opinions,
and which has no opinion on the subjects of opinion in general?

Certainly not.

But surely we are assuming a science of this kind, w