What is Nonsense?

"What is Nonsense? I know when you do not ask me. I know that in infancy it is as the very air we breathe; that it cheers and strengthens us in the long weary working days of manhood; and brightens and gladdens our old age. But how can I bring it within the words of a definition? If the question is pressed, i must answer it with another. What is Sense?"

Edward Strachey, in Edward Lear's "Complete Nonsense" introduction