This great purple butterfly,
In the prison of my hands,
Has a learning in his eye
Not a poor fool understands.
Once he lived a schoolmaster
With a stark, denying look;
A string of scholars went in fear
Of his great birch and his great book.
Like the clangour of a bell,
Sweet and harsh, harsh and sweet.
That is how he learnt so well
To take the roses for his meat.
William Butler Yeats
http://www.poemhunter.com/poem/another-song-of-a-fool/
