  The essence of special relativity is hiding in the equal existence of
particle and antiparticle, which can be expressed by two discrete symmetries
within one inertial frame --- the invariance under the (newly defined)
space-time inversion (${\bf x}\to -{\bf x},t\to -t$), or equivalently, the
invariance under a mass inversion ($m\to -m$). The problems discussed are: the
evolution of the $CPT$ invariance into a basic postulate, an unique solution to
the original puzzle in Einstein-Podolsky-Rosen paradox, the reduced Dirac
equation for hydrogenlike atoms, and the negative mass paradox leading to the
prediction of antigravity between matter and antimatter. {\bf Keywords}:
Special relativity, Reduced Dirac Equation, Antiparticle, Antigravity
