Michael Angelo said, one paints, not with one's hands, but with one's brain. Leonardo shocked the Prior of the Convent Delle Grazie by standing for days together opposite the Last Supper without touching it with the brush. The painter is a painter, because he sees what others only feel or catch a glimpse of, but do not see. We think we see a smile, but in reality we have only a vague impression of it, we do not perceive all the characteristic traits from which it results, as the painter perceives them after his internal meditations, which thus enable him to fix them on the canvas. The illusion is less easy as regards musical expression; because it would seem strange to everyone to say that the composer had added or attached notes to the motive, which is already in the mind of him who is not the composer. As if Beethoven's Ninth Symphony were not his own intuition and his own intuition the Ninth Symphony.
What is the underlying theme of the passage?