Leo Tolstoy, in Book II, Part Two, Chapter 1 of War and Peace delivers a perfect metaphor as a character wrestles with the meaninglessness of his life:
... No matter what he turned his thoughts to, he always came back to these same questions, which he could neither solve nor cease asking himself. It was as if in his head the main screw that held his life together was stripped. The screw would neither go in nor come out, but went on turning in the same groove without catching, yet it was impossible to stop turning it.
(from the Ann Dunnigan translation, 1968; see also TruthInBattle (11 Feb 2001), OozeOnVerst (22 Sep 2004), IrresistibleAttraction (4 Oct 2004), InfiniteSky (15 Oct 2004), UntutoredVoice (3 Nov 2004), ...)