  After analyzing the implication of investigations on the C, P and T
transformations since 1956, we propose that there is a basic symmetry in
particle physics. The combined space-time inversion is equivalent to
particle-antiparticle transformation, denoted by ${\cal PT=C}$. It is shown
that the relativistic quantum mechanics and quantum field theory do contain
this invariance explicitly or implicitly. In particular, (a) the appearance of
negative energy or negative probability density in single particle theory --
corresponding to the fact of existence of antiparticle, (b) spin- statistics
connection, (c) CPT theorem, (d) the Feynman propagator are linked together via
this symmetry. Furthermore, we try to derive the main results of special
relativity, especially, (e) the mass-energy relation, (f) the Lorentz
transformation by this one ``relativistic'' postulate and some
``nonrelativistic'' knowledge.
