  We review how the kinematic structures of special relativity and quantum
mechanics both stem from the relativity principle, i.e., "no preferred
reference frame" (NPRF). Essentially, NPRF applied to the measurement of the
speed of light $c$ gives the light postulate and leads to the geometry of
Minkowski spacetime, while NPRF applied to the measurement of Planck's constant
$h$ gives "average-only" projection and leads to the denumerable-dimensional
Hilbert space of quantum mechanics. These kinematic structures contain the
counterintuitive aspects ("mysteries") of time dilation, length contraction,
and quantum entanglement. In this essay, we extend the application of NPRF to
the gravitational constant $G$ and show that it leads to the "mystery" of the
contextuality of mass in general relativity. Thus, we see an underlying
coherence and integrity in modern physics via its "mysteries" and the
fundamental constants $c$, $h$, and $G$.
