Not all the singers of a thousand years

Not all the singers of a thousand years
Can open English prisons. No. Though hell
Opened for Tracian Orpheus, now the spell
Of song and art is powerless as the tears
That love has shed. You that were full of fears,
And mean self-love, shall live to know full well
That you yourselves, not he, were pitiable
When you met mercy's voice with frowns or jeers.

And did you ask who signed the plea with you?
Fools! It was signed already with the sign
Of great dead men, of God-like Socrates,
Shakespeare and Plato and the Florentine
Who conquered form. And all your pretty crew
Once, and once only, might have stood with these.