Vague, indistinct, avoiding outline, the phrase _un non so che_ leaves definition to the instinct of those who feel, but will not risk the limitation of their feeling by submitting it to words.
Nothing in antique psychology demanded a term of this kind.

Classical literature, in close affinity to sculpture, dealt with concrete images and conscious thoughts.

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.