Clorinda is about to march upon her doom.  She is inflamed with the ambition to destroy the engines of the Christian host by fire at night; and she calls Argante to her counsels: 

    Buona pezza e, signor, che in se raggira
    Un non so che d’insolito e d’audace
    La mia mente inquieta; o Dio l’inspira,
    O l’uom del suo voler suo Dio si face (xii. 5).

Thus at this solemn point of time, when death is certainly in front, when she knows not whether God has inspired her or whether she has made of her own wish a deity, Clorinda utters the mystic word of vague compulsive feeling.

Erminia, taken captive by Tancredi after the siege of Antioch, is brought into her master’s tent.  He treats her with chivalrous courtesy, and offers her a knight’s protection: 

    Allora un non so che soave e piano
    Sentii, ch’al cor mi scese, e vi s’affisse,
    Che, serpendomi poi per l’alma vaga,
    Non so come, divenne incendio e piaga (xix. 94).

At that moment, by the distillation of that vague emotion into vein and marrow, Erminia becomes Tancredi’s slave, and her future is determined.

These examples are, perhaps, sufficient to show how Tasso, at the turning-points of destiny for his most cherished personages, invoked indefinite emotion to adumbrate the forces with which will contends in vain.  But the master phrase rings even yet more tyrannously in the passage of Clorinda’s death, which sums up all of sentiment included in romance.  Long had Tancredi loved Clorinda.  Meeting her in battle, he stood her blows defenseless; for Clorinda was an Amazon, reduced by Tasso’s gentle genius to womanhood from the proportions of Marfisa.  Finally, with heart surcharged with love for her, he has to cross his sword in deadly duel with this lady.  Malign stars rule the hour:  he knows not who she is:  misadventure makes her, instead of him, the victim of their encounter.  With her last breath she demands baptism—­the good Tasso, so it seems, could not send so fair a creature of his fancy as Clorinda to the shades without viaticum; and his poetry rises to the sublime of pathos in this stanza: 

      Amico, hai vinto:  io ti perdon:  perdona
    Tu ancora:  al corpo no, che nulla pave;
    All’alma si:  deh! per lei prega; e dona
    Battesmo a me ch’ogni mia colpa lave. 
    In queste voci languide risuona
    Un non so che di flebile e soave
    Ch’al cor gli serpe, ed ogni sdegno ammorza,
    E gli occhi a lagrimar gl’invoglia e sforza (xii. 66).

Here the vague emotion, the non so che, distils itself through Clorinda’s voice into Tancredi’s being.  Afterwards it thrills there like moaning winds in an Aeolian lyre, reducing him to despair upon his bed of sickness, and reasserting its lyrical charm in the vision which he has of Clorinda among the trees of the enchanted forest.  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: