The aria for alto, oboe d’amore, and continuo is one of the finest pieces in all of Bach. The metaphor is sleep—mortal and eternal, that is, the body and the soul. It seems to me—cautious as one should be about reading anything into music, and almost everything into Bach’s—a profound and almost overwhelmingly heartfelt expression of desire for what would be best to be in human affairs, and cannot be realized, a quiet and ardent sadness that can only be vanquished by death.
There is a moment in the central section, of judgment, when the quick light of the end has come and gone, when Bach introduces a bridge, in the tempo and style of the opening section, to the da capo of that section. But he transfers the shadow of observation found in the line for oboe d’amore for a mere three bars to the violin, and the shadow becomes aspiration that is both reached and given.
There is another example of this, in the third song, Beim Schlafengehn (when falling asleep) of Richard Strauss’s Vier Letze Lieder, written in 1948. As the narrator sinks into sleep, after the end of all work and thoughts, a solo for violin takes the narrator from sleep to eternity, where the soul has rises in its flight to glide and sway and become free in the endless and bright dark of the magic realm that this art conveys and desires to reveal, im Zauberkreis der Nacht / tief and tausendfach zu leben, deeply and thousandfold to live.
This movement is also highly reminiscent of the tenor aria, also in siciliano rhythm, in the Michaelmas cantata 19, where it is a plea for the presence of the angels and our ennoblement by salvation.