The SI base units, or Systéme International d'unités, consists of the metre, kilogram, second, ampere, kelvin, mole and candela.
Many of these quantities are related to each other by various physical laws, and as a result the units of a quantities can be generally be expressed as a product of powers of other units; for example, momentum is mass multiplied by velocity, while velocity is distance divided by time.
Therefore the SI base units are no longer necessary but were retained because for historical and practical reasons.
[4] Duff argues that only dimensionless values have physical meaning and all dimensional units are human constructs.
The ontological issue is whether these fundamental constants really exist as dimensional or dimensionless quantities.
For instance, time and distance are related to each other by the speed of light, c, which is a fundamental constant.