Where our ancestors worked in complex visual, tactile and social settings, alert to subtle opportunities or threats, a clerk manipulates a handful of simple symbols on a featureless field. And while a dropped berry is of little consequence to a gatherer, a missed digit can invalidate a whole calculation. The peripheral alertness by which our ancestors survived is a distraction to a clerk. Clerking is hard work more because of the preponderance of human mentation it must suppress than the tiny bit it uses effectively. Ripples Like little ripples on the surface of a deep, turbulent pool, calculation and other kinds of procedural thought are possible only when the turbulence is quelled.
Humans achieve quiescence imperfectly by intense concentration.
Numbers are better manipulated as calculus stones or abacus beads than in human memory. A few cogwheels in Blaise Pascal's seventeenth century calculator perform the entire procedure of addition better and faster than a human mind. Charles Babbage's nineteenth century Analytical Engine would have outcalculated dozens of human computers and eliminated their errors. Such devices are effective because they encode the bits of surface information used in calculation, and not the millions of distracting processes churning the depths of the human brain. The deep processes sometimes help.