Precision is defined not as absolute perfection but as the tolerance: the permitted deviation between the actual and the intended dimension, expressed numerically. A part made to a tolerance of one-thousandth of an inch is more precise than one made to a hundredth, and this measurable gap, not craftsmanship or beauty, is what separates precision engineering from mere skilled handwork.
James Watt's steam engine failed because no one could bore a cylinder round enough to stop steam leaking past the piston, until in 1776 John Wilkinson adapted his cannon-boring machine to produce a cylinder accurate to within the thickness of a worn shilling across its diameter. This single advance in tolerance made the practical steam engine possible and is positioned as a founding moment of the industrial age.
The book argues that interchangeable parts, and therefore assembly-line mass production, became possible only once components could be manufactured to tolerances tight enough that any one part would fit any assembly without hand-fitting. This shift moved skill out of the individual craftsman and into the machine and the measurement standard itself.
Industrial and technological progress can be read as a steadily tightening sequence of tolerances, from Wilkinson's fractions of an inch to the nanometre and atomic scales of GPS, jet engines and computer chips. Viewing invention through the decreasing size of permissible error reframes progress as a quantifiable pursuit of exactness rather than a story of isolated inventors or ideas.