He stands before the cypress where the soul of his dead lady seems to his misguided fancy prisoned; and the branches murmur in his ears: Fremere intanto udia continuo il vento Tra le frondi del bosco e tra i virgulti, E trarne un suon che flebile concento Par d'umani sospiri e di singulti; E un non so che confuso instilla al core Di pieta, di spavento e di dolore (xiii.

40).
The master word, the magic word of Tasso's sentiment, is uttered at this moment of illusion.

The poet has no key to mysteries locked up within the human breast more powerful than this indefinite _un non so che_.
Enough has been said to show how Tasso used the potent spell of vagueness, when he found himself in front of supreme situations.

This is in truth the secret of his mastery over sentiment, the spell whereby he brings nature and night, the immense solitudes of deserts, the darkness of forests, the wailings of the winds and the plangent litanies of sea-waves into accord with overstrained humanity.

It was a great discovery; by right of it Tasso proved himself the poet of the coming age.
When the _Gerusalemme_ was completed, Tasso had done his best work as a poet.