Sleep is, first of all, a filter, or sieve.  It strains off the impressions that engross, but not enrich us,—­that superfluous material of experience which, either from glutting excess, or from sheer insignificance, cannot be spiritualized, made human, transmuted into experience itself.  Every man in our day, according to the measure of his sensibility, and with some respect also to his position, is mobbed by impressions, and must fight as for his life, if he escape being taken utterly captive by them.  It is our perpetual peril that our lives shall become so sentient as no longer to be reflective or artistic,—­so beset and infested by the immediate as to lose all amplitude, all perspective, and to become mere puppets of the present, mere Chinese pictures, a huddle of foreground without horizon, or heaven, or even earthly depth and reach.  It is easy to illustrate this miserable possibility.  A man, for example, in the act of submitting to the extraction of a tooth, is, while the process lasts, one of the poorest poor creatures with whose existence the world might be taunted.  His existence is but skin-deep, and contracted to a mere point at that:  no vision and faculty divine, no thoughts that wander through eternity, now:  a tooth, a jaw, and the iron of the dentist,—­these constitute, for the time being, his universe.  Only when this monopolizing, enslaving, sensualizing impression has gone by, may what had been a point of pained and quivering animality expand once more to the dimensions of a human soul.  Kant, it is said, could withdraw his attention from the pain of gout by pure mental engagement, but found the effort dangerous to his brain, and accordingly was fain to submit, and be no more than a toe-joint, since evil fate would have it so.  These extreme cases exemplify a process of impoverishment from which we all daily suffer.  The external, the immediate, the idiots of the moment, telling tales that signify nothing, yet that so overcry the suggestion of our deeper life as by the sad and weary to be mistaken for the discourse of life itself,—­these obtrude themselves upon us, and multiply and brag and brawl about us, until we have neither room for better guests, nor spirits for their entertainment.  We are like schoolboys with eyes out at the windows, drawn by some rattle of drum and squeak of fife, who would study, were they but deaf.  Reproach sleep as a waste, forsooth!  It is this tyrannical attraction to the surface, that indeed robs us of time, and defrauds us of the uses of life.  We cannot hear the gods for the buzzing of flies.  We are driven to an idle industry,—­the idlest of all things.