Strategies for the definition of a system of units

Quantity units give an empirical reference to numerical values in mathematical equations that represent physical laws. Units have then the role of relating theory and experiment, and a system of units is a critical component of what metrology has to offer to empirical sciences. We analyse here the strategic problem of the optimal criteria – in a sense to be specified – to define a system of units, where such a system is specified when (a) a system of quantities and (b) a unit for each quantity in the system are defined. For the definition of a system of units, the previous knowledge of a system of quantities is then assumed, as typically provided by physics or a subfield of it. The focus is on the strategies according to which quantity units can be defined. Hence what follows applies not only to the International System of Units (SI), in its current version (BIPM, 2006) and its possible future revision (BIPM, 2014), but also and more generally to the definition of any system of units, and it is aimed at discussing and providing a strategic perspective to such definition.