Then, I said, be cheerful, sweet sir, and give your opinion in answer to the question which I asked, never minding whether Critias or Socrates is the person refuted; attend only to the argument, and see what will come of the refutation. I think that you are right, he replied; and I will do as you say. Tell me, then, I said, what you mean to affirm about wisdom. I mean to say that wisdom is the only science which is the science of itself as well as of the other sciences. But the science of science, I said, will also be the science of the absence of science. 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 does not know, and to see what others know and think that they know and do really know; and what they do not know, and fancy that they 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? 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.