Re: It's all relative
Except counting "angels" is exactly the point of this exercise and other SI defining exercises.
Thus the unit of time (seconds) changed from a fraction of a day to a period in which a certain type of atom oscillates a specific number of times.
The metre was originally defined as a fraction of the Earth's polar circumference at the longitude of Paris, which embarrassment was corrected by making a stick the same length out of an invariant material and calling that the standard metre. Recently it was redefined as the distance light travels in a vacuum in 1/299 792 458 of a second.
Mass was first defined around a specific volume of a liquid notorious for the variability of it's properties. Cue another correction by making a rock out of the same stuff they made their stick. Problematically, this rock and others made using it as a model now fail to agree with each other as to what a kilogram is.
So work is underway to calculate the exact number of atoms in a perfect crystal of isotopically pure silicon polished into a perfect sphere in order to redefine mass as a function of length (ie. the distance between atoms in a crystal) just as length itself was itself redefined as a function of time and a physical constant c.
The end result is the three primary units of measurement, duration, distance and mass are now related to the characteristic behaviour of one atom under photonic stimulation, the distance a photon can travel while our one atom does it's thing so many times, and the number of a different type of atom that can be packed into a volume possessing a single defining characteristic (radius) that can be related to our photon's itinerary.
The idea is/was to do away with reliance on external events and macroscopic physical models which might be altered or destroyed. Good luck finding a toolkit more basic than 2 atoms and a photon.