Dr.Johnson seemed to his biographer, to show, by this ready answer, the acuteness of his wit and discernment.

But did not the wit consist in adroitly excusing himself, by an illusory comparison?
What analogy is there between the things which he compares?
Of the difficulty of defining _poetry_, and the difficulty of defining _light_, the reasons are as different as are the two things themselves, _poetry_ and _light_.

The former is something so various and complex that it is hard to distinguish its essence from its accidents; the latter presents an idea so perfectly simple and unique that all men conceive of it exactly in the same way, while none can show wherein it essentially consists.

But is it true, that, "We all know _what light is_ ?" Is it not rather true, that we know nothing at all about it, but what it is just as easy to tell as to think?
We know it is that reflexible medium which enables us to see; and this is definition enough for all but the natively blind, to whom no definition perhaps can ever convey an adequate notion of its use in respect to sight.
13.

If a person cannot tell what a thing is, it is commonly considered to be a fair inference, that he does not know.