Why Sir, it is much easier to say what it is not. We all know what light is; but it is not easy to tell what it is.'
Source: When asked, 'What is poetry'. Life of Johnson, (J. Boswell), Vol. III
-- Samuel Johnson, (Sep 18 1709-1784), English lexicographer, critic; He was remembered for writing the first critique of Shakespeare, 1765 and Dictionary of the English Language, 1755.