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?