The mediaeval art of Dante, precisely, mathematically measured, had not felt the need of it.  Boccaccio’s clear-cut intaglios from life and nature, Petrarch’s compassed melodies, Poliziano’s polished arabesques, Ariosto’s bright and many colored pencilings, were all of them, in all their varied phases of Renaissance expression, distinguished by decision and firmness of drawing.  Vagueness, therefore, had hitherto found no place in European poetry or plastic art.  But music, the supreme symbol of spiritual infinity in art, was now about to be developed; and the specific touch of Tasso, the musician-poet, upon portraiture and feeling, called forth this quality of vagueness, a vagueness that demanded melody to give what it refused from language to accept.  Mendelssohn when some one asked him what is meant by music, replied that it had meanings for his mind more unmistakable than those which words convey; but what these meanings were, he did not or he could not make clear.  This certainty of sentiment, seeming vague only because it floats beyond the scope of language in regions of tone and color and emotion, is what Tasso’s non so che suggests to those who comprehend.  And Tasso, by his frequent appeal to it, by his migration from the plastic into the melodic realm of the poetic art, proved himself the first genuinely sentimental artist of the modern age.  It is just this which gave him a wider and more lasting empire over the heart through the next two centuries than that claimed by Ariosto.

It may not be unprofitable to examine in detail Tasso’s use of the phrase to which so much importance has been assigned in the foregoing paragraph.  We meet it first in the episode of Olindo and Sofronia.  Sofronia, of all the heroines of the Gerusalemme, is the least interesting, notwithstanding her magnanimous mendacity and Jesuitical acceptance of martyrdom.  Olindo touches the weaker fibers of our sympathy by his feminine devotion to a woman placed above him in the moral scale, whose love he wins by splendid falsehood equal to her own.  The episode, entirely idle in the action of the poem, has little to recommend it, if we exclude the traditionally accepted reference to Tasso’s love for Leonora d’Este.  But when Olindo and Sofronia are standing, back to back, against the stake, Aladino, who has decreed their death by burning, feels his rude bosom touched with sudden pity: 

    Un non so che d’inusitato e molle
    Par che nel duro petto al re trapasse: 
    Ei presentillo, e si sdegno; ne voile
    Piegarsi, e gli occhi torse, e si ritrasse (ii. 37).

The intrusion of a lyrical emotion, unknown before in the tyrant’s breast, against which he contends with anger, and before the force of which he bends, prepares us for the happy denouement brought about by Clorinda.  This vague stirring of the soul, this non so che, this sentiment, is the real agent in Sofronia’s release and Olindo’s beatification.