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.