WikiMechanics uses touching the Earth as a reference sensation. So we presume that there are some feelings to do with life on Earth that are known to just about everyone; for example, feeling the pressure of bodily contact with the floor.
In principle, we imagine that all such experiences are composites of Anaxagorean sensations so that the Earth can be considered as a huge compound quark. The number of quarks in this composite is extremely large but finite. Because N is so large, it would require a rare, catastrophic event like a planetary collision to make a significant change to the distribution of the quarks. So, accordingly we consider that the covariance between components of their inertia is constant.

