Nothing in nature says an inch must be the length it is
Almost every unit we use, from the inch to the mile, is a historical deal rather than a law of nature. Measurement laws began as tools against cheating traders. Only recently have the world's base units been freed from physical objects and tied instead to unchanging constants of physics.
To measure something is to compare an unknown quantity with a known reference of the same kind and express how big or small it is. Every measurement can be described by four features: its type, its magnitude, its unit and its uncertainty. The type is the kind of comparison involved, whether by ratio, by difference or by simple ranking. The magnitude is the number read from an instrument, the unit scales that number against a standard, and the uncertainty captures random and systematic errors, estimated by repeating measurements and weighing an instrument's accuracy and precision.
Fields disagree on what counts. In physics and engineering, following the metrology vocabulary published by the International Bureau of Weights and Measures, purely nominal properties are not measured at all. Statistics and the social sciences, by contrast, recognise several levels, from nominal and ordinal up to interval and ratio scales.
For most of history, units were fixed by local agreement between trading partners. From the 18th century, efforts to unify them led to the International System of Units, which builds every physical measurement from seven base units: the kilogram, metre, second, ampere, kelvin, mole and candela. None now depends on a physical artefact that could decay or be destroyed. The first person to suggest such an artefact-free standard was Charles Sanders Peirce, who proposed defining the metre by the wavelength of a spectral line; Michelson and Morley cited and refined his method.
Treaties hold the system together. The General Conference on Weights and Measures, created by the Metre Convention of 1875, redefined the metre via the speed of light in 1983 and the kilogram via the Planck constant in 2019. Even the yard is metric underneath: in 1960 the United States, Britain, Australia and South Africa agreed it equals exactly 0.9144 metres. National bodies such as NIST in the United States and the National Physical Laboratory in Britain police commercial measurement.
Source: Measurement
