  Dirac's equation with static external fields is considered as an equation of
relativistic quantum theory, which describes the quantum dynamics of an
electron in four-dimensional space-time. It is shown that the Dirac electron
has only one intrinsic degree of freedom -- spin. The quantum ensemble of an
electron with a given energy consists of two inseparable from each other
subensembles of "heavy" and "light" spin-1/2 quasiparticles to have opposite
(relative) intrinsic parities. In the case of electrical scalar potential
nonuniform in the direction of a particle motion, these subensembles behave as
analogs of the electrical and magnetic components of the plane monochromatic
electromagnetic wave propagating in a dispersive medium, nonuniform in the
direction of a wave motion. The role of states with negative energies and the
properties of the velocity operator are explained.
