Congruence problem

Measurement: SI-units (SI = System International d'Unités)

Length (metre. m)
Mass (gram, g)
Time: duration (second, s)
(Electr.) current (ampere, A)
(Thermodyn.) temperature (kelvin, K)
Amount of substance (mole, mol)
the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
Luminous intensity (candela, cd)

Weber-Fechner (versus Stevens).

Measurement of sense perception: starting with a smallest unit, and then using steps - (a sort of qunatization). It leads to logarithmic scales (e.g. decibel scale for noise); in contrast to the power functions of SYtevens. General Problem: ordering in size is easy, to attach numbers is difficult. Compare different metrics for something as innocent as R.